Ed Milibands Shadow Cabinet is split over whether to support the High Speed 2 rail project, casting major doubt on the viability of the £50 billion scheme.
While Mr Miliband is a supporter of HS2, Andy Burnham, the Shadow Health Secretary, is one of four shadow ministers to break ranks and criticise the plan in damning terms.
Mary Creagh, the Shadow Transport Secretary, is also under intense pressure from her local party to withdraw Labours support for the scheme, while others in the Shadow Cabinet are privately opposed to HS2.
Senior party sources suggested that Labour could scrap the plan or slow down the construction timetable to avoid the need for difficult cuts to public spending if Mr Miliband wins next years election.
Cross-party support for HS2 is seen as essential because it requires a long-term commitment of taxpayer funds before it is completed in 2033.
The latest disclosures will fuel concerns among Conservative MPs that Labours support for HS2 is soft and that the party is poised to ditch the project, leaving David Cameron saddled with an unpopular and expensive policy.
At last years Labour conference, the Shadow Chancellor, Ed Balls, warned that there would be no blank cheque from his party, a statement that caused panic in Whitehall that Labour support was wavering.
The Prime Minister subsequently challenged Mr Miliband to come clean, warning that without cross-party backing, the essential new fast line linking London with the North would collapse.
Mr Milibands office responded by formally pledging the partys support and ordering Labour MPs to vote in favour of the Bill paving the way for the detailed proposals to move forward.
However, shadow cabinet ministers have now raised fresh objections to the plans.
Mr Milibands close ally Michael Dugher, the shadow Cabinet Office Minister, Yvette Cooper, the Shadow Home Secretary, Jon Trickett, the deputy party chair and Mr Burnham have criticised HS2 in formal submissions to the consultation on the new route.
All are under significant pressure over HS2 in their constituencies, which will be affected by the second phase of the proposed line, which runs from Birmingham to Manchester and Leeds.
Mr Burnham, the MP for Leigh in Greater Manchester, said that while he could see a case for HS2 overall, he cannot support the governments current plans for his region.
They deliver maximum disruption and minimal benefit to an area which has amongst the poorest access to rail transport of any in the country, Mr Burnham said.
The project would involve a depot in his constituency and would seriously damage the natural landscape of Leigh and Wigan, which is only slowly recovering from its history of coal mining and heavy industry.
HS2 needs to go back to the drawing board on its proposed depot, he said.
It is clear to me that the Governments current proposal for HS2 in the North West fails to maximise its potential benefit to our region and its taxpayers, he wrote.
Whilst acknowledging the need for progress and infrastructure to devolve economic power from London to the wider regions, it cannot be allowed to be at the expense of local communities, whose greatest resource is the green space in which they reside.
To maximise the opportunity of a major infrastructure plan for my constituents, I am only able to offer my support to the HS2 proposal if my concerns over the impact on residents and the need for a station to serve the people of Leigh are implemented.
Last week, Mr Balls said that final approval for the Parliamentary Bill setting out the detailed work for the London to Birmingham phase one of HS2 was likely to be delayed until after the next election because the plans are so complex.
This led to fears that it would be easier for Labour to cut or scrap the project if the party returned to power next year.
One shadow cabinet source said: Ed Balls is highly sceptical about HS2 and others are as well.
Miss Cooper, who is married to Mr Balls, has also written to the government to warn of her constituents concerns over the project. She criticised the inadequate consultation process and poor information provided to residents by HS2 Ltd and the government.
Miss Cooper also suggested changes to the route. It is understood that her submission did not state one way or the other whether she supports HS2 in general.
Mr Trickett, the shadow minister without portfolio and MP for Hemsworth in West Yorkshire, is also said to have made a submission which criticises the governments consultation process.
Mr Dugher, the MP for Barnsley East, said the proposed phase two route passed directly through his constituency. Understandably, the proposed route is a matter of great concern to me and to many of my constituents, he said.
He said he was supportive in principle of HS2 but raised specific concerns in the consultation over the value for money of the project as well as other local issues.
If this consultation shows there is overwhelming public opinion that the route should be altered, then HS2 should listen and look at proposals to do so.
One shadow cabinet source said Labour could scrap the project or scale it back after the election.
I do not support HS2, the source said. This is a huge amount of money. It has to be shown that it is better value for money than other rail and road infrastructure projects.
The source predicted that the question of the partys support for the project will become a live issue at the next election.
Scrapping the project or slowing it down could release billions of pounds for other policies and avert the need for sensitive spending cuts elsewhere.
I do not believe that the Treasury have produced a proper business case. If Labour wins the election, that work will have to be done and it is hard to see how it would be convincing, the source said.
While Miss Creagh, the MP for Wakefield, has publicly supported HS2, her local Labour council has voted strongly to oppose the plan.
Councillor Jean Askew said: We are totally against it. We would much prefer the money to be spent on the infrastructure within our network.
The MPs have to listen to us, she said. We put them where they are. Mrs Askew claimed that Miss Creagh personally supported their objections.
Maureen Cummings, another Labour member of Wakefield Council, said she had gathered 1,000 signatures for a petition against HS2 from her ward alone.
Mrs Cummings said she disagreed with Miss Creaghs policy of supporting HS2. She is the Shadow Transport Minister and she has her views. I would like her to support the residents of our district because the residents of our district have quite clearly said that they are not convinced about HS2.
Miss Creagh, the shadow transport secretary, insisted that "Labour supports HS2 because we must address capacity problems".
"Labour MPs are rightly engaging with HS2 on how the line can deliver the best economic and social benefits to their local communities," she said. "The Government must use this consultation properly to address local concerns and to make sure that those affected are properly compensated."
Andrew Bridgen, a Conservative MP opposing HS2, said he feared that Labour were preparing to abadon the project shortly before next year's election, leaving the Tories with a "damaged and discredited" policy on polling day.
"This is cynical politics by Labour because HS2 is already costing taxpayers £1 million a day," he said.
