Hundreds of new train carriages could remain out of action until 2006 because of a power shortage.

South Central, which operates most train services in Sussex, has ordered 700 new carriages to replace its 40-year-old slam-door trains.

But only 42 are being used because the power supply serving rail lines in the South-East desperately needs upgrading.

The new carriages - which feature air conditioning, sliding doors and computerised controls - consume more power than the electricity supply can provide.

Railtrack planned to upgrade the power supply in the late Nineties but work was constantly delayed.

Responsibility has passed to the Strategic Rail Authority, which has made improving the supply a priority amid fears it could take two years longer than expected.

Porterbrook, the train leasing company, estimates only 200 of the carriages ordered by South Central last March can be used before work is carried out.

The power failure also affects Connex South Eastern, which has ordered 600 carriages, and South West Trains, which has ordered 785.

The Government's ten-year transport plan includes a target of replacing all slam-door trains in the Southern Region with 2,200 safer carriages by 2005.

The problems with the carriages were first noticed in Kent when Connex South Eastern found its new 12-car trains were breaking down.

But Wendy Toms, chairman of the Rail Passengers' Council for Southern England, feared Sussex and Hampshire would be worst affected.

She said: "This will be an enormous and expensive task for the SRA, covering miles and miles of track.

"It is important they tell us where they think the deadline can be met.

"Then the train companies will have time to refurbish the old rolling stock so it can be used for a bit longer.

"Why did no one realise the trains would consume more power? This is yet another example of the problems brought by fragmentation of the railways, with no one taking responsibility."

The SRA plans to begin awarding contracts for the £1 billion work over the next six weeks.

A South Central spokeswoman said they had been able to introduce new carriages on some services between Seaford and Portsmouth.

She added: "There has been an issue regarding the power supply.

"A project group is being set up, led by the SRA, to look at delivering extra power.

"We are committed to phasing out our slam-door carriages as soon as possible."