A multi-million pound stockpile of modern train carriages will continue to lay idle for another year while passengers make do with aged rolling stock.

South Central, which operates the Brighton to London service and coastal routes in Sussex, has spent tens of millions of pounds on the trains but has been forced to mothball them because there is not enough power in the tracks to put them into service.

The company has 100 of the £1 million carriages in storage - with more arriving every week - while commuters are forced to make do with 40-year-old slam-door trains that should have been pensioned off.

The National Audit Office (NAO), the Government spending watchdog, has highlighted the situation in a damning new report.

It says: "Delays in delivering new trains - ranging from a month to more than two-and-a-half years - and making essential upgrades to the power supply and other infrastructure on Network Rail's Southern Region (the work did not start until mid-2002), mean the December 2004 deadline for taking all of the oldest slam-door trains out of service is unlikely to be met.

"Some 300 new carriages might be ready to enter service in early 2004 but will be prevented from doing so if essential infrastructure upgrades are not completed by then."

Sir John Bourn, head of the NAO, said: "While some passengers are enjoying the improved facilities and rides provided by new trains, many others are waiting for new trains to enter service.

"Too often the trains turn out to be unreliable in everyday use because they haven't been sufficiently tested. The Strategic Rail Authority needs to redouble its efforts to help get new trains into service on time."

South Central has 700 of the Class 377 Electrostar carriages - worth £856 million - on order from Bombardier, the Midlands-based manufacturer.

A spokesman for South Central said the firm had 172 new carriages in use but 100 were in storage outside Sussex.

The Strategic Rail Authority is working on a £1 billion scheme to boost power supplies but it is not known when this will be completed.

The spokesman said there was no firm date for when the final slam-door train would disappear.

He added: "Passengers will see slam-door trains disappearing regularly for the rest of the year."