Commuters could find themselves paying an £800 million bill for replacement trains on the London Underground Central Line.

They are among thousands of Tube users across London who may have to subsidise new trains, after suffering months of disruption, because a range of defects means the whole fleet may have to be replaced much sooner than expected.

The London Assembly Transport Committee has heard that, despite modifications after the Chancery Lane derailment, trains are being withdrawn from service for repairs.

Passengers using the Central Line stations in Wanstead and Woodford face months of rush hour chaos with around an eighth of trains being withdrawn for safety reasons every day.

Assembly members now fear that if the problems plaguing the service continue, the fleet may have to be replaced sooner than the 30 years they are intended to operate for.

They are concerned that, because of the high cost of keeping the trains in service, Metronet and Tube Lines Infracos, responsible for the maintenance of the trains, tracks, signals, tunnels and stations, may be tempted to make short-term repairs until its contact is renewed in 2010.

Lynne Featherstone, chairwoman of the committee, said: "Replacing all the Central Line trains may seem far fetched but neither Metronet nor Transport for London (TfL) could give us a firm assurance that this would not happen.

"There is no way Metronet will want to pay £800 million to replace them. It's in its interest to keep the trains going somehow until 2010 and at the first contract review, allow this huge bill to fall to Londoners."

Assembly members fear Metronet could also argue that the various problems with the trains invalidate the contract it signed to run the services on nine lines in London.

A Metronet spokesman claimed this scenario was unlikely to happen and said: "There are no plans to renew the Central Line trains. They are only 10 years old.

"We acknowledge there have been some problems with them because they have a history but we are putting that right and have committed ourselves to a major engineering programme."

But London Underground's managing director Tim O'Toole told the committee the possibility of a 'get out' for Metronet on such a major piece of investment had always concerned him.