Lord Mandelson has urged the government to ditch the controversial HS2 rail link, saying it will be "an expensive mistake" that could "damage" the northern regions it is intended to help
The former Business Secretary admitted Labour's enthusiastic backing of the scheme in 2010 was "partly politically-driven" in the run up to that year's general election, and conceded that the party had not given sufficient consideration to alternatives.
With the estimated cost of the line, which is intended to speed up rail connections between London, Birmingham, Manchester and Leeds, rising to £42.6 billion, the Labour peer said that those - including himself - who previously backed the plan "should not offer it an open cheque and should, instead, insist on keeping their options open".
While Labour's leadership remains supportive of the high-speed rail link, disquiet within the party about the scheme was exposed at the weekend when former chancellor Alistair Darling declared himself an "HS2 sceptic".
Writing in the Financial Times today, Lord Mandelson said: "I once supported High Speed 2, a proposed rail link from London to the North that the Labour government of which I was a member first put forward. There are no simple options when it comes to transport - but I now fear HS2 could be an expensive mistake."
Labour's plans were based on an estimated cost of £30 billion and on a central assumption - that its construction would spread growth across the country - which was "neither quantified nor proved", said the peer. And he acknowledged that Gordon Brown's government was at that point driven by the search for major infrastructure projects which might inject vitality into the economy in the wake of the financial crash.
Alternatives, such as upgrading the east and west coast main lines, regional rail improvements and mass transit systems in provincial cities, "were not actively considered" by the Labour administration as it gave the green light to HS2.
"In 2010, when the then Labour government decided to back HS2, we did so based on the best estimates of what it would involve," said Lord Mandelson. "But these were almost entirely speculative.
"The decision was also partly politically driven. In addition to the projected cost, we gave insufficient attention to the massive disruption to many people's lives construction would bring. Why? Not because we were indifferent but because we believed the national interest required such bold commitment to modernisation."
And he concluded: "By all means, let the cost-benefit analysis, and even the paving legislation currently going through Parliament, continue.
"But all the parties - especially Labour - should think twice before binding themselves irrevocably to HS2. It is not all it seems and has the potential to end up a mistake, damaging in particular to those people that it was intended to help."
Additional reporting Press Association