The expected full cost of the project has also soared from £62bn to £88bn at today’s prices.

The latest revelation that costs have spiralled and the project is already seriously behind schedule is a major blow to the industry as it faces the independent Oakervee review on whether the project should now go-ahead at all or be scaled down.

This is due to report to the Government this autumn.

Shapps said that enabling works presently employing a 9,000-strong workforce could continue while a verdict is awaited.

The first phase was due to open at the end of 2026, but it could now be between 2028 and 2031 before the first trains run on the route.

The second phase to Manchester and Leeds was due to open in 2032-33, but that has been pushed back to 2035-2040.

Shapps revealed that HS2 chairman Alan Cook had provided him with the latest update on taking up his job as transport secretary.

He said: “There is no future in obscuring the true costs of a large infrastructure project – as well as the potential benefits.”

He said that Cook believed benefits of the current scheme were substantially undervalued and that HS2 would continue to refine its estimates of cost, benefits and schedule.

Shapps added: “During the short period in which the independent review completes its work I have authorised HS2 Ltd to continue the current works that are taking place on the project.

“This will ensure we are ready to proceed without further delay for the main construction stage of Phase 1 in the event that the government chooses to continue.

“Similarly, I intend to continue to progress the next stages of the hybrid Bill for Phase 2a, West Midlands to Crewe, in the House of Lords while the review is ongoing.”

A spokesman for the High Speed Rail Industry Leaders group said: “A major infrastructure project of this nature will have its timescales and costs updated periodically, and this has not happened on HS2 since 2015.

“We must not lose sight of the importance of HS2 to the mission of joining up Britain.

“The project is essential, and irreplaceable, to the Government’s goal of fixing the north-south divide which has beset Britain for generations.

“Already, it has triggered massive new private investment in Birmingham, and plans are well advanced in cities like Manchester and Leeds too. All of this would be scuppered by any decision to cancel or curtail HS2.”