Labour will support remaining in a customs union with the European Union after Brexit, Sir Keir Starmer has said.
The shadow Brexit secretary warned it was now "crunch time" for Theresa May over her approach to the customs union, and said it would be "better" to reach "bold" new trade agreements by working with the EU.
Sir Keir said Labour had "long championed being in a customs union with the EU and the benefits".
He told BBC One's The Andrew Marr Show: "Obviously it's the only way of realistically to get tariff free access, it's really important for our manufacturing base and nobody can answer the question how you keep your commitment to no hard border in Northern Ireland without a customs union."
Sir Keir said Labour had had "many weeks of discussion unanimously" and had agreed to develop their policy, to be announced by the Labour leader Jeremy Corbyn on Monday.
He continued: "The customs arrangements at the moment are hardwired into the membership treaty so I think everybody now recognises there's going to have to be a new treaty - it will do the work of the customs union.
"It's a customs union, that's what the CBI are saying now, it's what the various amendments are now all saying - there's going to have to be a new agreement, but will it do the work of the current customs union? Yes, that's the intention."
Sir Keir said Britain was more likely to strike new deals if it works "jointly with the EU", adding: "We all want to do bold new trade agreements but we would be better off doing that with the EU."
He also signalled support for cross-party amendments on the customs union to the Trade Bill.
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 and the majority in Parliament needs to be heard and it will be heard sooner rather than later."