Former Business Secretary Lord Mandelson has said Labour originally approved the High Speed Two rail project to "upstage" the Conservatives.
He told the House of Lords his party "did not feel like being trumped" over creating a fast north-south link by the Tories, then in opposition.
Lord Mandelson said he now thought there was "flimsy evidence" to back the multi-billion-pound project.
But the government insists it will boost the UK economy.
HS2 is intended to allow trains to run at 250mph (400km/h) from London to Birmingham from 2026, with branches to Manchester and Leeds via Sheffield planned for 2033.
The estimated cost of the plan has risen in the past few months from £34.2bn to £42.6bn - plus £7.5bn for rolling stock.
Gordon Brown's government debated the issue before deciding in favour.
Lord Mandelson said: "Frankly, there was too much of the argument that if everyone else has got a high-speed train we should have one too... We didn't feel like being trumped by the then opposition's support for the high-speed train. We wanted, if anything, to upstage them."
Lord Mandelson was a keen supporter of HS2 when he was in government, but earlier this year suggested the project should be scrapped.
He told the Lords that Labour's front bench was right to have become "more sceptical", adding: "I have been an ardent pro-railway supporter all my adult life. But it is precisely for that reason that I do not support HS2.
"I think the sheer cost of it will suck the very life blood out of the rest of the country's rail system."
The Labour peer said Sir Rod Eddington's report on transport in 2006 had been forgotten. It had "firmly rejected" HS2 and concluded that a "wide range of incremental improvements" would be better.
He also said HS2 was a "political trophy project justified on flimsy evidence to be about modernity and prosperity".
But Liberal Democrat Lord Greaves had backed the project as "sensible, necessary and long overdue".
He said the rail scheme would herald a new era for Britain's railways and help close the "regional divide".
And Labour former minister Lord Rooker hit back at Lord Mandelson's criticism of HS2, saying: "The Labour government started this project and it will be inconceivable to withdraw support.
"I am getting cheesed-off listening to ex-ministers swanning around the political salons pouring cold water on this project."
In response to Lord Mandelson's comments, a Downing Street spokesman said: "The prime minister's position on HS2 is well stated. We need to invest in high-speed rail in this country.
"It is essential for solving the capacity crisis and research from KPMG has shown there could be a £15bn annual boost to the economy.
"The prime minister is clear in his support for HS2 and wants the process to go on and the railway to be built."