The Transport Secretary will confirm tomorrow that HS2 could cost £100billion despite it being scaled back several times.

Heidi Alexander will tell the Commons that delivering the beleaguered high-speed rail project may cost triple the £32.7billion price tag it was originally given in 2011.

And this will only cover the remaining planned stretch connecting London with Birmingham.

The original route would have seen new tracks continue onto Leeds and Manchester after forking into an Eastern and Western leg northwards from Birmingham.

But these were scrapped by previous Tory governments to save cash as costs rocketed. It suggests that delivering the original route would have comfortably exceeded £150billion.

Ms Alexander’s oral statement to MPs tomorrow will be the first time officials have officially acknowledged costs for the London-Birmingham route could hit up to £100billion, the equivalent of £1billion a mile.

She is expected to set out a range for the projected final cost, with £100billion being at the top end.

She will confirm that the top speed for the line will be reduced to 320kmh (199mph), from the originally planned 360kph (224mph), to cut costs further.

It means HS2 will no longer be the fastest trainline in the world, but will still have trains as fast as Japan, Morocco and India.

The fastest trains are in China and Indonesia, where services can hit 350kmh (217mph).

Conventional trains in the UK run at speeds of up to 200kph (124mph). The definition of high-speed covers trains running at 250kph or more.

Insiders said the latest move would only extend journey times between London and Birmingham, currently projected to be 41 minutes, by around three to five minutes because trains were rarely meant to reach 360kph. The fastest route with Avanti West Coast is currently around 75 minutes.

Ms Alexander is also expected to push back the timetable yet again for when the first trains could run, from 2033 to at least 2035.

The latest review setting out what has gone wrong with the project’s delivery will be published alongside her speech and will blame rocketing costs on the ‘gold-plating’ of the original design with too much focus on ‘the highest possible speeds’ which resulted in ‘bespoke and highly-engineered design’.

Costs were also driven up by starting construction work ‘at the hardest points of the route’, such as London Euston, it will add.

Other ‘original sins’ identified in the report, by former National Security Advisor Sir Stephen Lovegrove, include constantly ‘changing objectives and political priorities’, levels of ‘costs and risk being very badly underestimated’ and contracts being awarded too early in the design process.

The review, partly based on interviews with senior officials involved in the project, will also say that HS2 and Department for Transport staff ‘felt under significant pressure from ministers to keep things moving’ despite warnings about rocketing costs.

Tory MP for Mid Buckinghamshire, Greg Smith, whose constituents have been blighted by HS2 building works for several years, said: ‘Nothing surprises me anymore with HS2, it is the deep hole that keeps swallowing up our cash for no purpose whatsoever.

‘My constituents suffer daily at the hands of this monster and to see their taxes go up and up and up to fund its ever increasing bill is adding insult to injury.’

Lord Tony Berkeley, who served as the deputy chair of a government-ordered review into HS2, has been warning for several years that costs could exceed £100billion.

He said: ‘I feel completely vindicated.

‘They’ve batted on regardless and spent taxpayers’ money like there’s no tomorrow.

‘I’m afraid it’s too late now to pull the plug. You can’t leave half-finished concrete structures across the Chilterns.

‘But whether they’ll learn any lessons will be interesting to see.’

The project has already burnt through more than £40billion of taxpayers' cash.

HS2 CEO Mark Wild will be quizzed by MPs on the Commons transport committee on Wednesday about the project’s progress.