It comes after The Independent reported ministers were considering shelving the northern phase amid concerns about spiralling costs and severe delays.

The Prime Minister's official spokesman said "spades are already in the ground on our HS2 programme and we're focused on delivering it" but would not promise the line would go to Manchester.

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"I can't comment on speculation around a leaked document. It is obviously standard process for departments to discuss the phasing of major projects like HS2... but the work is already under way," he said.

Asked whether the prime minister was committed to the line going to Manchester, the spokesman said: "We are committed to HS2, to the project.

"I can't comment on the speculation that's a result of a photograph. We are as you know looking at the rephasing of the work in the best interests of passengers and taxpayers."

Signs that the leg to the northern city may be in trouble came when the DfT confirmed in March that work on the crucial leg between Birmingham and Crewe - which is then due to continue to Manchester - would have to be put on hold because of the impact of inflation.

It meant that services were not going to extend to Manchester until the 2040s.

In its report, The Independent said a cost estimate revealed that the government has already spent £2.3bn on stage two of the railway from Birmingham to Manchester, but that ditching the northern phase could save up to £34bn.

The newspaper said the documents were discussed at a meeting in Downing Street on Tuesday and suggested the £2.3bn was now not recoverable even if it is cancelled.

HS2 plagued by delays

Delivery of the high-speed railway has been a core pledge of the Conservative government but it has been plagued by delays and ever-increasing costs.

The initial opening date of 2026 has fallen back to 2033, while cost estimates have spiralled from about £33bn in 2010 to £71bn in 2019 - excluding the final eastern leg from the West Midlands to the East Midlands.

It is not just the northern section of the project that has encountered trouble, with there also being doubts about the future of Euston station in London.

Under the HS2 map, Euston is supposed to provide the final leg in the south after linking up to Old Oak Common in the suburbs of northwest London.

However, Transport Secretary Mark Harper recently told MPs that work at Euston would be paused for two years because costs had risen to £4.8bn compared with an initial budget of £2.6bn - which MPs on the Public Accounts Committee described as " wildly unrealistic".

Since construction began there six years ago, hundreds of homes and businesses have been demolished - but now the huge building site is virtually empty.

The DfT has said it will use the two-year period to determine its priorities and minimum requirements for the station.

This summer HS2 was also rated "unachievable" as part of an annual report from a government watchdog.

The Infrastructure and Projects Authority (IPA) gave the first two phases of the rail link - from London to Birmingham, and from Birmingham to Crewe, a "red" rating, saying the whole plan may need to be "reassessed".

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The government announced further delays to HS2 earlier this year in an effort to "balance the books" after inflation hit the cost of materials.

But analysis exclusively leaked to Sky News showed the two-year pause in works would end up being three-and-a-half years, and was likely to cost the taxpayer at least £366m.

In its report, the IPA said: "Successful delivery of [HS2] appears to be unachievable."

The watchdog added there were "major issues with project definition, schedule, budget, quality and/or benefits delivery, which at this stage do not appear to be manageable or resolvable".

As a result, it said the project "may need re-scoping and/or its overall viability reassessed".

This latest criticism comes in the same month that HS2 Ltd's chief executive, Mark Thurston, announced he would be resigning from his post in September.