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HS2 could cost up to £22 billion more than its previous budget and may be delayed by seven years, the company building the railway has said.

A report by HS2 Ltd chairman Allan Cook stated that the original plans "did not take sufficient account" of the effect of building a high-speed line through areas that are densely populated and with challenging ground conditions.

The railway was due to open in phases, with the final sections from Crewe to Manchester, and Birmingham to Leeds, completed by 2033. But the scheme is not expected to be completed until as late as 2040, Mr Cook added.

Mr Cook added that the project's budget has "proved unrealistic", but added: "At the same time the benefits have been understated."

He said the previous budget for HS2 was £55.7 billion but it could cost up to £78 billion, with both figures at 2015 prices.

In a written statement to Parliament, Transport Secretary Grant Shapps said: "Colleagues will see that the chairman of HS2 does not believe that the current scheme design can be delivered within the budget of £55.7 billion, set in 2015 prices.

"Instead he estimates that the current scheme requires a total budget - including contingency - in the range of £72 to £78 billion, again in 2015 prices.

"Regarding schedule, the chairman does not believe the current schedule of 2026 for initial services on Phase One is realistic.

"In line with lessons from other major transport infrastructure projects, his advice proposes a range of dates for the start of service.

"He recommends 2028 to 2031 for Phase One - with a staged opening, starting with initial services between London Old Oak Common and Birmingham Curzon Street, followed by services to and from London Euston later.

"He expects Phase 2b, the full high-speed line to Manchester and Leeds, to open between 2035 and 2040."

The project had previously been branded a “white elephant project” by Tory former minister David Davis, and there have been growing calls for the Government to scrap it.

But in his statement on Tuesday, Mr Shapps said that “HS2 is the single largest project of this government" and said the UK must invest in modern infrastructure.

He said: “We all in this House know we must invest in modern infrastructure to ensure the future prosperity of our country and its people.

"We look back to past achievements with a sense of pride – from the canals and railways that ensured the UK led the world into the Industrial Revolution, to the space ports and launch sites we are now considering that will make the UK a global leader in space.”

Responding to the announcement on Tuesday, a spokesman for HS2 Ltd said: "The report by our chairman Allan Cook is an assessment of the current status of the HS2 programme.

"It examines the comprehensive and far-reaching benefits of HS2 for the country, which are more significant than those previously taken into account.

"It also examines the challenges facing the project. It sets out a rigorous cost and schedule range for delivery of the programme under its current scope.

"The assessment makes clear that HS2 remains a compelling strategic answer for Britain's future transport needs, relieving overcrowding and congestion on our roads and railways, and reducing the carbon footprint of the UK.

"It will drive economic growth and regeneration in our regions, and bring Britain closer together.

"The assessment of Phase One is based on the significant work carried out so far, which has given us detailed insight into the scale and complexity of the programme. The assessment of Phase 2b takes into account lessons learnt on Phase One."