Transport Secretary Ruth Kelly launched a robust defence of the Government's rail record but acknowledged problems experienced over New Year had been "serious and unacceptable".

The Bolton West MP told the Commons last night that the the Office of Rail Regulation was to extend its investigation into the engineering overrun and promised lessons would be learned from the process.

Ms Kelly said: "Over Christmas and the New Year period there were serious and unacceptable delays on key sections of the rail network, including at Rugby and Liverpool Street Station.

"I very much regret the impact this has had on thousands of passengers and the chief executive of Network Rail has rightly apologised.

"He has also told me that he intends to learn all the lessons from the engineering works delays to minimise the risk of unforeseen over-runs in the future."

Earlier, the Tories accused ministers and Network Rail of a "monumental foul-up" over the severe disruption for New Year travellers.

Shadow transport secretary Theresa Villiers blamed the Government and Network Rail's "incompetent" senior management for the chaos.

Ms Villiers said that combined with rail fares increases, passengers had endured a "truly miserable New Year greeting from this Government and Network Rail".