Labour is ready to team up with Tory rebels to force Theresa May to agree a customs union with the EU, Sir Keir Starmer has signalled.
The Brexit Secretary said "crunch time is coming for the Prime Minister" as he claimed Ms May did not have enough support in Parliament to push through plans to leave the vital EU trade agreements.
Ahead of a major Brexit speech by Jeremy Corbyn on Monday, Sir Keir confirmed the party had "unanimously agreed" to back staying in a customs union with the EU after Brexit.
He told the BBC's Andrew Marr Show: "The customs arrangements at the moment are hard-wired into the membership treaty so I think everyone now recognises there has to be a new treaty. It will do the work of the customs union so it's a customs union."
Sir Keir indicated that Labour would support rebel amendments to a key Brexit bill tabled by pro-EU Tories Anna Soubry and Ken Clarke, which would put customs union membership back on the table.
He said: "The Labour front bench put down a number of amendments paving the way for the option of a customs union - they went down a few weeks ago.
"Now these cross-party amendments have gone down essentially saying the same thing and to put it bluntly crunch time is coming for the Prime Minister."
Asked whether Labour would back the cross-party amendments, he said: "We haven't made a final decision on that but they are so close to our amendments ... but whether it's our amendments or cross-bench amendments, crunch time is now coming for the Prime Minister because the majority of Parliament does not back her approach to a customs union.
"The majority in Parliament needs to be heard and it will be heard sooner rather than later."
The amendment to Ms May's trade bill is shaping up to be a crucial flash point, as a growing number of Tory MPs are likely to support remaining in a customs union with the EU.
It is also backed by the small group of Tory "mutineers" who inflicted an embarrassing defeat on the Government last year by winning a meaningful vote for MPs on the final deal.
Eurosceptic cabinet minister Liam Fox urged Tory Remainers to keep an "open mind" as he suggested Ms May's Brexit war cabinet on Thursday has agreed proposals that would deal with their fears.
The International Trade Secretary told The Andrew Marr Show: "I would say to my colleagues that Theresa May has kept a broad range of views on the European issue for a reason.
"We sat down with those differing views, we set out the issues, we looked at the options and we came to an agreement that we are all happy with."
He added: "I hope that they will have an open mind and listen to what the Prime Minister says because I think that what the Prime Minister will set out will deal with a lot of the reservations that they have."
The next stage of the bill has been pushed back until April, partly due to attempts by government whips to manage the rebellion.
Asked if legislation was being delayed because the Government would lose, Dr Fox replied: "We want to persuade our colleagues of the merits of our argument before we take the bill forward."
If passed, the amendment would make it an “objective of an appropriate authority to take all necessary steps to implement an international trade agreement which enables the UK to participate after exit day in a customs union with the EU in the same terms as existed before exit day”.