Last year Robert Goodwill, the HS2 minister, said: “We are considering whether to open the service to Old Oak Common a few months early, and push back the completion of Euston for a few years.”

This was later ruled out, but could now be back on the table.

The announcement of the precise route for phase 2 is already running about a year behind schedule. It was to have been decided last autumn, but HS2 now says it will be announced in “autumn 2016,” subject to the Heywood review.

Whitehall sources said the preference was to try to “re-establish control,” if possible, over the costs of the existing scheme without major cuts to its scope.

In real terms, HS2 will cost up to nine times more per mile than the French TGV high-speed lines, according to the House of Lords’ economic affairs committee.

One key opponent of HS2 said he had recently been called in by senior Treasury officials to tell them his concerns about the scheme.

“They said they wanted to see me urgently. They even offered to come to me,” he said.

“They said they were fully in favour of HS2 but they were worried about it. They were listening very intently when I said that the project was out of control.”

HS2 has already spent more than £1 billion before the legislation to build it has passed through Parliament and before a single foot of track has been laid.

Large sums of money have been paid in “facility fees” to local residents for allowing access to their land or property. One opponent of HS2 said she had received 17 separate visits, collecting payments of between £160 and £1,000 for each visit.

HS2 is also taking over four floors of a prestigious new office development in central Birmingham at a rent of at least £2.8 million a year.

Lord Berkeley, chairman of the Rail Freight Group, which represents goods users on the railway network, said HS2 “hasn't got a clue” what its costs are and now faces a “watershed” as the legislation to build it passes through a select committee of the House of Lords chaired by a former Supreme Court judge.

“The Lords committee is, I think, going to treat things very differently [from the Commons],” he said.

“It’s going to be almost like a court.”

Lord Berkeley said he had been told that HS2’s “governance” was the focus of the Heywood review but added: “It is a mess at the moment. I personally worry the whole thing’s going to get cancelled.”

The HS2 bill is supposed to have passed Parliament by October but is unlikely to have done so.

The Government is trying to speed the process by attempting to ban more than 600 individuals and groups from making representations to the Lords committee, but this will be challenged by many of those affected in hearings next month.

As well as its problems with costs, HS2 has been savagely criticised by a parliamentary committee and its own internal review for major failings in its engagement with residents affected by the scheme.

In a previously little-publicised report, MPs on the public administration select committee attacked HS2’s “culture of defensive communication and misinformation,” saying it was “not acceptable…within a public body responsible for the delivery of such a large and highly controversial project.”

HS2 also commissioned an internal review by Ian Bynoe, a former police complaints commissioner.