Expensive new rolling stock is breaking down more often than old slam-door trains run by South Central, a report reveals today.

The National Audit Office said that by September last year the new trains were travelling an average of 19,000 miles before being hit by problems.

The old ones which are still being replaced had a reliability record of 30,000 miles.

The new stock has suffered mechanical failures, glitches with on-train computer software and problems with air conditioning and power supply.

The NAO said in March 2003, four months after the new trains were introduced, that South Central's reliability record was even worse. Trains were covering an average of only 9,000 miles before breaking down.

In a damning report, the spending watchdog said the picture was bleak across the rail network.

Half the train operating companies which introduced new stock suffered a slump in reliability.

Lack of capacity to test trains before they entered service was blamed.

The NAO also seized on delays in introducing new rolling stock.

South Central has admitted many of the 736 carriages ordered to replace its 40-year-old slam-door stock will be late into service.

The new coaches - with air conditioning, sliding doors and computerised controls - are heavier and use more power than the electricity supply can provide.

The NAO's report, Improving Passenger Services Through New Trains, said there were no figures for South Central trains alone.

But taxpayers could face a £7 million bill because about 300 new carriages ready to enter service nationwide early this year could not because of delays in upgrading infrastructure.

Edward Leigh, chairman of the House of Commons Public Accounts Committee, said passengers would see how badly the introduction of new trains had been managed and called it "a catalogue of blunders so far".