Concerns are growing that the true cost of HS2 could eventually be more than £100bn, almost double the amount the Government has based its economic case on.
The high speed line already has a medium value for money rating based on the Department for Transort's traditional WebTAG methodology.
The analysis carried out for the appraisal of HS2 in the Economic Case found that the project had a benefit-cost ratio of 2.3 - for every £1 spent on the project would generate a benefit worth £2.30. However this includes wider economic impacts.
Prime minister Boris Johnson has suggested recently in an interview that the final cost could be more than £100bn - not the £56bn previously tabled - but that he would still 'hesitate' to cancel the scheme.
'I do think it's only responsible as an incoming government, with all the controversy surrounding the spend on HS2, which will probably be north of £100bn, it's only responsible to have a short review without interrupting the timetable at Curzon Street or anywhere else,' Mr Johnson said.
The prime minister is also establishing a review into the project to make sure the business case still works.
It is understood the Mr Johnson is keen to woo northern voters by arguing for the second phase of the high speed line - a two-branch section from Birmingham on to Manchester and Leeds - to be built first.
A recent written parliamentary question asking for current estimates on the total cost of the project also raised eyebrows with its non committal response - a change of message after the Government had previously stuck fast to the £56bn figure.
This time transport minister Baroness Vere of Norbiton said: 'HS2 Ltd continues to update its cost estimates as the project develops. In relation to HS2 Phase One, we will make these details public as part of the business case which will inform Notice to Proceed later in 2019.'
An HS2 Ltd spokesperson declined to answer questions on what the current cost estimate is.
The spokesperon said: 'Our chair continues to scrutinise the programme, and we’re aware the prime minister has also said he will commission a review.
'HS2 is hugely important for future infrastructure needs of the country, and we’re determined to ensure we have a project that rebalances the economy and provides value for money for the taxpayer.'