Sir John Bourn, the head of the National Audit Office, said that contrary to Gordon Brown, he believes the public would have to carry the costs of any collapse by Network Rail.

He told the Commons Treasury sub-committee: "If Network Rail goes down we are not going to have no railways in country. The government will provide them."

The chancellor has argued that Network Rail's £21 billion debts should be excluded from Treasury books as it counts as a private company.