The HS2 high-speed rail project could cost as much as £106 billion, according to an official government review not yet published.

The review led by Doug Oakervee, a former chairman of HS2, reportedly says there is “considerable risk” that the project’s cost will rise by up to 20% beyond the £81 billion to £88 billion range set out in a report by current HS2 chairman Allan Cook just four months ago.

Mr Oakervee’s review says work on phase 2b of HS2 from the West Midlands to Manchester and Leeds should be paused for six months to study if it could mix conventional and high-speed lines, according to the Financial Times, which has seen a copy of the paper.

The review says the Government should “on balance” continue with the 250mph railway, which would initially go from London’s Euston station to Birmingham and then to Leeds and Manchester by 2040, but that this is subject to “a number of qualifications”.

How Birmingham's HS2 hub has been imagined. Credit: HS2

It says there needs to be “further work” done to assess HS2’s impact on regional growth and it is “hard” to say what economic benefits will result from building it.

“Transport investment alone will not ‘rebalance’ the UK economy,” the review adds.

Prime Minister Boris Johnson is expected to decide within weeks whether to go ahead with construction on the first phase of what would be Europe’s largest infrastructure project.

The review said savings to the £106 billion figure could be made by having the private sector contribute to funding HS2 stations, lowering specifications and improving the “cost performance of the management”.

It also says that no other high-speed line in the world runs 18 trains per hour and recommends reducing it to 14.

£8 billion has already been spent on the HS2 project.

An HS2 Ltd spokesperson said: "HS2 Ltd is developing our organisation to make it simpler, more flexible and more efficient as we move into the construction of Phase One."

They added: "This work is entirely unrelated to, and pre-dates, the Oakervee Review and is standard practice for infrastructure delivery bodies as they move from the planning stage to delivery.

"We will be working closely with the staff affected as roles and responsibilities change.

"In every case we will first explore redeployment and mitigating options, with redundancy as a last resort."