The Turbo Star trains, which cost �2.5m each, have been running between London St Pancras and Sheffield for less than a week.

The BBC's Sarah Sturdey: "Customers complained to staff that they were getting rained on"

But manufacturer Adtranz was called in to repair all 17 trains after passengers on the Midland Mainline service complained they were getting wet.

A spokesman from the service said: "The problem came to light on Wednesday when we had two-thirds of June's average rainfall in one day.

"The trains were doing over 100mph and some water came dripping through one of the seals.

"Unfortunately it hadn't been possible to replicate that pressure of rain with that speed in the factory, but we are now taking steps to modify the seals."

Good news

The spokesman added: "Three of the trains have already been converted and the rest of the fleet should be completed in the next few days.

"It's a shame, really, because it clouds the good news. The new fleet has cut journey times from Sheffield to London by nine minutes and has allowed us to double the number of journeys."

The blunder comes less than a week after it emerged that sliding-door commuter trains built by Derby-based Adtranz were too wide for more than 20 stations in south-east England.

Railtrack said the cost of trimming the edges of platforms to allow trains to fit would be about �1m - although Per Staehr, UK chief executive of Adtranz, said Railtrack had approved the new trains before they were built.