Labour should adopt a wholehearted pro-EU stance and become the party of the 48% who voted Remain at the EU referendum, according to Christian Wolmar.
The Labour candidate for the Richmond Park and North Kingston by-election told IBTimes UK that he would not vote for Article 50, the official mechanism to split from Brussels, unless the government offered MPs clarity on the issue.
"I can't possibly vote for a policy that I know is bad for Britain, that would be irresponsible," he said. My view is that we ought to be the party of Remain, we ought to be the party of the 48% and build on that."
The transport author, 67, argued that the EU had made "terrible mistakes", including the Euro currency, but warned that the UK government should not be given a "blank cheque" for a Brexit.
"I could not vote for a one line statement that says we have to invoke Article 50 with no explanation," Wolmar said.
"We can't sign-up to a blank cheque on this. Parliament is sovereign, that's in the constitution."
The comments come after England's High Court ruled that MPs must have a vote on the mechanism after the government unsuccessfully argued ministers had the legal authority to trigger Article 50 without giving parliament a say.
Theresa May's administration is challenging the decision at the Supreme Court from 5 December, with a final ruling expected in January 2017.
But Shadow Chancellor John McDonnell signalled on 15 November that Labour would not block a vote on Article 50 in the House of Commons. The left-winger claimed such a move could put Labour on the side of "corporate elites".
IBTimes UK pressed Wolmar on McDonnell's remarks: "John is not in disagreement with the idea that we want to know what Brexit means before voting on it. I think that's the party position," he said.
"My position, which is pretty much in accordance with it, is that I could not vote for a one line statement that says we have to invoke Article 50 with no explanation."
The Labour candidate added: "The referendum was an advisory referendum on the basis of a lot of lies like the £350m for the NHS."
Wolmar's stance puts him in line with pro-EU Liberal Democrat hopeful Sarah Olney and in direct opposition to Brexit campaigner and former Conservative MP, Zac Goldsmith.
Why Labour are standing in the seat
Goldsmith triggered the by-election in October when he quit the Commons over the government's decision to back a third runway at Heathrow Airport, a move Olney and Wolmar also oppose.
Goldsmith is running as an independent, while the Tories, Greens and Ukip are not contesting the seat. Ukip have endorsed Goldsmith and the Greens have endorsed Olney.
Labour, meanwhile, secured just over 12% of the vote in Richmond Park at the 2015 general election and a BMG Research poll, of more than 500 voters between 26 and 27 October, put the party on 10% in the seat, with Goldsmith on 56% (-2) and Olney on 29% (+10).
The Liberal Democrats are billing the contest as a two-horse race. But Wolmar defended his inclusion, arguing that there are substantial policy differences between Labour and the Liberal Democrats, such as the former's anti-austerity stance.
"The other parties joined together in a coalition that brought about austerity," he said.
"The Liberal Democrats agreed to this crazy economic policy that led to hundreds of thousands of jobs being lost. An economic policy that doesn't work. At a time of low interest rates and high unemployment, you don't then have austerity. The government should spend money on infrastructure and the like."
Wolmar added: "There's a good bedrock of support here, the Liberal Democrat vote is very soft. It mostly consists of people who don't want Zac or don't want the Tories and would very happily vote Labour or Liberal Democrat."
The Richmond Park and North Kingston by-election will be held on 1 December.