Former chancellor Alistair Darling has come out against HS2, warning ministers their plans to give Britain's transport infrastructure a major boost could become a "nightmare".
Darling, who also served as transport secretary while in government, used a Times article to speak out against the proposals to cut journey times between London, Birmingham, Manchester and Leeds.
His comments will put pressure on Ed Miliband's shadow Cabinet to reappraise its position of cautious support for the project as the party approaches its autumn conference in Brighton next month.
"One thing I have learnt is that transport, rather like banking, is at its best when it is boring," Darling wrote.
"That is when it tends to work. Political visions can easily become nightmares."
He attacked the economic case for HS2, questioning the benefits the Department for Transport claims will result from the improved infrastructure and warning that the projected cost could spiral out of control.
Treasury officials now reportedly believe the cost of HS2 could reach £70 billion in cash terms, significantly more than the £30 billion price Darling endorsed while chancellor in 2010.
"The facts have changed," he added. "The case for HS2 was just about stateable in 2010. I don't believe that it is today.
"It is not too late to revisit the project. We need to ask ourselves what we would gain if it goes ahead. Equally we must then ask ourselves what we will have to lose."
He suggested England's biggest cities would not spend the money on HS2 if they were each handed £10 billion for economic development - and said other capital rail investment projects would lose out if HS2 goes ahead.
His intervention will reinvigorate the disquiet among Tory backbenchers against HS2 led by Amersham MP Cheryl Gillan, who was sacked from the Cabinet after threatening to resign over the issue.
"We have said consistently that the costs of HS2 will keep going up... how long will it be before the cost over £100 billion?" Stop HS2 campaign manager Joe Rukin said.
"There is a very simple reason as to why every new revelation about HS2 is bad: because it's a bad idea, poorly thought out, with no justification that stands up to any scrutiny and it's being implemented by buffoons."
The government is focusing its energies on maximising the economic benefits of HS2, with the establishment last month of an HS2 growth taskforce chaired by former Olympics chief Lord Deighton.
It argues HS2 will generate 22,000 jobs over the next five years and says this will rise to up to 50,000 jobs by the late 2050s.
"Building on the success of HS1 and its role in the transformation of Kings Cross, HS2 will act as catalyst to city centre regeneration and major development schemes," transport secretary Patrick McLoughlin said.
"HS2 is a huge project and I am determined to get maximum payback from the investment."
Under the government's plans trains will run at 225mph on the HS2 line, with 18 cities linked to HS2 services from 2033.
This is not the first time Darling has spoken out against HS2. Last month he told the Sunday Telegraph it would "suck money" out of the railway budget.