NR chief executive Ian Coucher also admitted he still did not know why up to half of the Jarvis specialist team had not turned up, more than three weeks into an internal inquiry.

A shortage of "overhead linesmen" meant engineering works on the West Coast mainline at Rugby, that should have been completed by December 30, overran by four days.

There was further chaos in London when late-running work at busy Liverpool Street station forced its last-minute closure ahead of the big return to work.

Network Rail has faced severe criticism and could face a massive fine from regulators on top of paying out £10 million compensation to passengers and train operating companies.

The Commons transport committee heard Jarvis had supplied a list of specialist overhead linesmen it would be using for the work at Rugby to allay fears about overruns.

In the event many simply did not turn up - up to half of the 70-strong team at one point - and attempts by Network Rail to recruit others at short notice proved fruitless.

So far however, Jarvis had not provided "a satisfactory justification", the MPs heard from a representative of Bechtel, the co-ordinating contractor for the work.

Asked if they would be penalised for the failure to provide the workers, Mr Coucher said: "They will get paid for some work but not all of it, but there's no penalty - we can't flow down the extra costs that we incur down to them."

Asked if that was a sensible approach, he said: "Normally they engage because they know failure would mean they wouldn't get any additional work. That's the risk they take." A variety of contracts were used, he added, some of which did involve penalties.