Previously supressed reports on HS2 raised major concerns the project would not be affordable.

Reports from the Major Projects Authority written in 2011 and 2012 were finally made public for the first time yesterday, after Transport Secretary Patrick McLoughlin had implemented the rarely-used power of veto to prevent their publication.

The 2011 report stated the review team "cannot judge with a high degree of confidence" that the "project is affordable".

And a year later those concerns were repeated by members of the Department for Transport.

The 2012 report said: "The Department believes however that the costs of this project are so large, and over such a long period, that it will not be able to afford it alongside all its other likely spending commitments."

It went on: "We are not convinced that there is a clear understanding as between HS2/DfT on the one hand and HM Treasury on the other as to the interpretation of these figures and confidence levels in them.

"A detailed 'whole of project' cash budget needs to be in circulation, which can then serve as the basis on which affordability discussions can take place. Without it there is a risk that the affordability risk, and the issue of where responsibility for this lies, is not properly addressed."

The previous report, written in 2011, said: "The Review Team has significant concerns over the readiness of the project to move into the development phase, subject to a positive decision, in particular the immaturity of the governance and management arrangements necessary for the development phase.

"The Review Team considers that the Department cannot judge with a high degree of confidence that the delivery of the London to West Midlands project is affordable. It is important that further work between DfT and Treasury continues on the ultimate affordability of the project through the development phase."

The report recommended it was "essential" the Department for Transport needed to "develop a clear financial control framework for the programme", with the report authors recommending this should take place within two months. It added it was "critical" that the team "should focus on completing the strategic and economic cases".

Both reports gave HS2 a red/amber rating, meaning successfully delivery of the scheme was in doubt with "major risks or issues apparent in a number of key areas".

It had been hoped the project would have Royal Assent in spring 2015, with the 2011 report noting confidence in hitting this target was "medium to low".

Mr McLoughlin used a veto to prevent the reports from being made public. A judicial review challenge against that decision was made in March, but the government withdrew from legal proceedings - allowing the reports to be published for the first time.