When your MG is stored for some length of time it may result in the clutch disk being stuck firmly to the flywheel and/or pressure plate so it will not disengage when the clutch pedal is depressed. This problem is derived from the great pressure applied to the clutch disk by the pressure plate. It is also aggravated by humidity and condensation, especially when the car is stored in an unheated garage subject to large temperature changes in a humid environment (typical winter storage). Some clutch disks may contain metallic particles that can corrode to adhere to the flywheel. Others might just leach out some of the bonding agents holding the friction material together to effectively glue itself to the flywheel. This makes it appear that the crankshaft is permanently connected to the input shaft of the transmission, since no amount of pumping the clutch pedal will cause the clutch disc to disengage. There are a few solutions to this problem and I suggest you try the easiest ones first.

The first order of business is to check to see if there might be some other problem preventing normal clutch operation. Start by reviewing CT-101 to be sure that the hydraulic slave cylinder is functioning properly. If it is not, then you have to fix that first. If the car lost the hydraulic fluid from the master cylinder the clutch slave cylinder may not be moving. The simple solution then may be to fill the master cylinder reservoir and bleed the clutch hydraulic line.

There are a few other things that could prevent clutch disengagement, but are not related to storage. Installing a clutch disk backward will make it bind on the flywheel mounting bolts in the center and will definitely prevent disengagement. A newly installed spigot bushing in the tail end of the crankshaft, if it is too small after installation, can seize on the gearbox input shaft to make it turn with the crankshaft even when the clutch does disengage. A delaminated clutch disk can result in pieces of the friction material multi-layered and jammed between the flywheel and pressure plate. These things can also prevent clutch disengagement, even when the hydraulics are working properly. The essence of this article assumes that the clutch was working okay when the car was parked or stored away, and is not working immediately after being retrieved from storage.

The simplest procedure can be done in your garage in a few seconds. It assumes that the brakes are working, that the clutch hydraulics are working, the starter motor works, and that the battery is charged. (I suppose it also rashly assumes that you have two feet and at least one hand). Sit in the driver's seat. Pump the brake pedal a couple of times and hold the brakes on firmly with your right foot. Depress the clutch pedal fully with your left foot. Put the transmission in 4th gear (least likely to drive the car away), and engage the starter for about one second. If you're lucky the torque from the starter motor may break the flywheel free from the clutch disk, and the engine will crank over normally. If that doesn't work in the first try or two, move on to Plan B.

Addendum, April 2010: This additional suggestion comes from Lindsay Sampford, Cambridgeshire, UK.
"Start the engine in neutral, let the engine run until it is well and truly warmed up, 15 minutes ought to do it. Then with the clutch pedal depressed, blip the throttle vigorously several times. The heat from the engine and the rapid change in engine speed will often break the seize, if it doesn't, proceed to [other] remedies. The heating certainly helps to free a stuck clutch, an old hand gave me this advice years ago and it worked for me".

Addendum, April 2010: This additional suggestion comes from M.J. (Mike) Moore, Cheshire, UK.
"What I did was to jolt the plate loose by running the engine at a fast tick-over whilst in neutral and with the clutch pedal fully depressed I quickly threw it into gear. I can't remember what gear I decided to throw it in but it worked first or second time with no ill effects as far as I can remember".

On this point I would NOT advise trying to jam it into 1st or reverse gears. If the clutch disk is stuck tight you might break some teeth on the straight cut non-synchronized gears. You could first test action of the synchronizers by pushing the shift lever gently toward engagement of 2nd, 3rd and 4th gears (with engine running). If the synchro is working properly you will not be able to push it into gear, and there will be no grinding noise. If the synchro is bad you may get a grinding noise, in which case you should not use that gear for this procedure.

Given you can find a good synchronizer (more likely 3rd or 4th gear), then you can try Mike's suggestion. Run engine to fast tick-over, depress clutch pedal and brake pedal (firmly), and pull shift lever hard toward engagement of the gear. If it actually goes into gear, you win, the clutch disc has broken loose. If you cannot get past the synchronizer, nothing lost, and proceed to other methods.

What we Yanks call synchronizer ring the Brits calk baulk ring ("baulk" meaning brake, as in friction brake). What happens here is that the baulk ring is actually putting brakes on the gearbox input shaft by pressing it into engagement with the output shaft. This is not a lot of braking force, but it may be enough to unstick the clutch disc.

The rest of these procedures require some open running space on a totally vacant street or empty parking lot. Put the transmission in neutral, start the engine and allow it to run up to normal running temperature. Give it a tune up if necessary so it runs reasonably well. Switch engine off, and take the car to a safe open space (preferably paved). You may expect the car to lurch forward during this operation, so be sure there is no one and nothing around the car. In fact for this procedure you will actually be driving the car.

With car stationary, engine off and parking brake off, depress the clutch and select 1st gear. Hold the throttle down slightly (1/2" or so), and start the engine. The car should jerk lurch forward and gradually pick up speed until the engine is running at or above idle speed. Run it up to about 2500 rpm, which will be about 10 mph in 1st gear. This engine speed is the speed at which a stock engine will develop near maximum torque. With the clutch pedal still fully depressed, and running at around 2500 rpm, alternate the throttle pedal between full down and full up position. This will alternately apply full engine driving torque and full engine braking torque (coasting down with no throttle) to the clutch, and with a little luck this may be sufficient to break the bond to free the clutch disk. Assuming that it will work, you should be prepared to get your foot off the throttle before you over-rev the engine, and get on the brakes to bring the car to a stop. If this does not work then take the car to a convenient open work space, kill the engine, and proceed to Plan C.

With the engine and parking brake off and the vehicle pointed in a safe direction, use a hydraulic floor jack with a long handle (not a bottle jack). Place the jack under the center differential housing of the rear axle to lift both rear wheels so they are clear of the ground by about two inches. Get into the car and confirm that there are no obstacles or people in front of the vehicle. With engine and parking brake still off, shift the transmission into 4th gear (least driving torque at rear wheels when full engine torque is applied up font). Start the engine and throttled up to a constant tachometer reading of about 1500 rpm. This puts a substantial amount of energy into the rotating engine and flywheel. Depresses the clutch pedal and keep it depressed, then apply the brakes firmly and suddenly. The inertia of the engine and flywheel will apply a very large torque to the stuck clutch, limited only by how fast and how hard you can apply the brakes. If this works the clutch will disengage and the engine will continue running when the brakes are applied. Then set the parking brake, remove the jack, and you can drive the car normally. If this doesn't work it will kill the engine. If it still doesn't work after a few tries, proceed to Plan D.

This procedure is a little more dramatic, and it requires an assistant. As in the prior method, raise the rear wheels a couple inches off the ground with a hydraulic floor jack. Leave the jack handle extending to the rear, and very close to the ground. The driver gets in the car, selects 4th gear (least torque at rear wheels), and starts the engine. This time run the engine up to about 2500 RPM, so it may develop good flywheel energy and maximum torque (if you hit full throttle). On prearranged signal, the assistant then snaps the jack hydraulic valve full open to drop the car suddenly to the ground. If all goes well the tires will suddenly stop turning while the engine keeps running, and the clutch bond will have been broken. If it doesn't work, the tires will squeak once, the car will lurch forward a bit, and it will kill the engine.

The functional difference between this method and simple application of the brakes is that the full weight of the car is brought to bear on the tires to provide sudden resistance on the drive shaft, which may allow more applied torque than you get by using the brakes. This is not guaranteed, as with well functioning brakes you should be able to lock it up and skid the tires when the car is moving. However, dropping the running car onto the rear tires may apply the torque more suddenly than you might do with the brakes, so the resulting shock to the clutch might be greater. Also, in addition to the torque you get from inertia of the engine and flywheel, if the partners in this venture can time things right, you might hit full throttle just at the same time that the tires come into contact with the ground. This will apply full engine torque to the clutch in addition to the torque resulting from sudden deceleration of the flywheel. As with the prior method you should assume that it will work, and be ready to get your foot off the throttle before over-revving the engine when it breaks loose.

If that doesn't work, take the car back to the garage and proceed with Plan E. Remove engine from car, remove clutch pressure plate and clutch disk from flywheel. Use large pry bar if necessary to pull pressure plate free. Use a solid punch (or chisel) and heavy hammer on the edge of the clutch disk if necessary to separate it from the flywheel. This would of course be the most extreme and least convenient of all, so lets hope one of the prior plans works first so you don't have to pull the engine.

Addendum February 19, 2014:
These tricks are courtesy of Bob Krzywicki in Arlington, VA, USA

I used a piece of 1" square tubing slipped over the throw out lever fork (the slave pushrod removed), used a strap and put it over the front hub, so when you turn the wheel the distance shortens and pulls the lever towards the front and the frame and cross member acts as a fulcrum.
I used a screw driver as a lever in the universal joint to turn the drive shaft, and I could turn the steering wheel to engage and disengage the clutch. At first I was able to turn the drive shaft while the car was in gear and the clutch fully disengaged and the engine fan would turn. Then after several moments of turning the drive shaft with the engine turning there was some slight noises, and then the fan did not turn. After that the drive shaft always turned with the clutch disengaged and the engine fan did not move. Turn the steering wheel to the right and the disk would engage and the fan would start to turn. So mechanically the clutch assembly is working fine and now it engages and disengages with predictability.

Addendum February 20, 2015:
On 2/15/2015, Robert Hine n England wrote:
"What we usually do is jack up the rear. Start it up in gear and let things warm up for ten minutes or so. Then with one foot pushing the clutch down to the floor, stamp on the brake pedal with the other foot. This has never failed to free up the stuck clutch".

On 4/22/2023, Geoff Piddington wrote:
"To make it easier, first run the garden hose into the bell housing and thoroughly soak the clutch assembly, allow the water to penetrate the rust and you will find it much easier. The water will evaporate as the engine warms up".