Jeremy Corbyn has committed Labour to supporting a customs union with the EU, a stance which raises the prospect of his party joining forces with Tory rebels to strike a major blow to the Government.
Speaking in Coventry, Corbyn said his party has "long argued that a customs union is a viable option for the final deal" and so a Labour government "would seek to negotiate a new, comprehensive UK-EU customs union to ensure there are no tariffs with Europe and to help avoid any need whatsoever for a hard border in Northern Ireland."
Corbyn added that his proposed deal would need to ensure that "the UK has a say in future trade deals" with third-party countries, unlike the present customs union, which gives Brussels control over all external trade agreements.
"Labour would not countenance a deal that left Britain as a passive recipient of rules decided elsewhere by others," Corbyn continued. "That would mean ending up as mere rule takers."
Corbyn's speech comes after Keir Starmer, the shadow Brexit secretary, hinted that Labour would support amendments by Tory Remainers Anna Soubry and Ken Clarke to Theresa May's flagship trade bill, set to return to the Commons within weeks.
Some have suggested the trade bill will become an issue for Theresa May, and that lawmakers could even bring down the Government by backing the amendments.
Asked if such a defeat would bring down the Government, shadow international trade secretary Barry Gardiner told BBC Radio 4's Today programme: "I don't know what it would lead to. I don't think anybody does."
But he added that if Brexit is about restoring sovereignty, it is right for Parliament to determine the policy.
Gardiner said: "We have said that we recognise the benefits of the customs union as it stands and we are now saying that a customs union between the EU and the UK, where we together decide those third-party countries where we will have common tariffs and common quotas, is of benefit."
Gardiner has previously said that a Turkey-style customs deal with the EU would be a "disaster" for the UK.
But he has now moderated his stance, saying that Labour wants a different approach that will give the UK a say over the access of third-party entrants to the customs union and will not leave Westminster being a "rule-taker" from Brussels.