Ukip has branded as "shabby politics" an alleged attempt by two Cabinet ministers to ban publication of a damning report on the controversial £50 billion HS2 rail project which would run between London, Birmingham, the North West and the North East.
And the Stop HS2 organisation said the blocking, allegedly by Transport Secretary Patrick McLoughlin and Cabinet Office Minister Francis Maude, was "absolutely disgraceful".
A Project Assessment Review of 2011 put HS2 in the amber/red category - a category which says successful delivery of the project is in doubt.
The documents from the report have yet to be published despite requests under the Freedom of Information (FoI) Act. One request was refused by the Department for Transport in June 2012 under a provision of the FoI Act, with the department ruling that "disclosure of the information would not be in the public interest".
Earlier this year the Information Commissioner ruled that the documents should be released but the Government has appealed against this decision, with a tribunal hearing due later this week.
However, according to the Daily Mail, Mr McLoughlin and Mr Maude have written to Prime Minister David Cameron urging him to put an emergency veto in place to prevent publication.
According to the Daily Mail, the letter says: "Continuing with the appeal would create political and presentational difficulties at a crucial point in the HS2 project's development . . . (therefore we should) exercise the veto now.
"Counsel has advised that we are very likely to lose the appeal. We consider that the importance the Government attaches to the successful implementation of the HS2 project . . . justifies the use of the veto in this case as exceptional."
It allegedly goes on: "Disclosure of such recent information would also have a chilling effect on assessments of other Government projects and, indeed, on advice prepared for Government ministers on many other subjects.
"Counsel has advised that it will be better to veto now rather than after an adverse tribunal decision."
Ukip deputy leader Paul Nuttall said: "The Project Assessment Review should be published, and it should be published now.
"The fact that we have Cabinet ministers seeking to block this report on HS2 demonstrates the cloak-and-dagger approach the Government is taking to this whole project and transparency in general.
"Rather than being open, frank and honest about the many potential difficulties that HS2 is going to face, the Government are seeking to bury that bad news. It is shoddy politics."
Stop HS2 campaign manager Joe Rukin said: "It is absolutely disgraceful that the Government doesn't want MPs, who should be fully informed before voting on the colossal expenditure HS2 entails, to actually have all the information about the project.
"This just proves that there has been, and continues to be an agenda from Government to deceive MPs and the public with spin and dismiss any independent information showing just how bad the HS2 project is."
Those requesting a comment from the Department for Transport were being referred to the Cabinet Office.
A Cabinet Office spokesman said: "This Government is proud to lead the world on transparency. We have published over 10,000 datasets on health, education, justice and transport to drive up the standards of public services and improve accountability.
"Earlier this year we published the first-ever Major Projects Authority annual report with project-level ratings for each of the Government's major projects."
HS2, whose £50 billion cost includes £7.5 billion for the high-speed trains, has a first phase which runs from London through Tory heartlands in the Chilterns to Birmingham which is due for completion in 2026.
A second, Y-shaped line north of Birmingham to north west and north east England is due to be finished around 2032/33.ajor projects."