The British Labour leader Jeremy Corbyn has confirmed his party's support for a new customs union with the EU after Brexit.
His comments mark the party's most concrete support for a 'soft Brexit' to date.
The move is likely to increase pressure on Theresa May, who has previously said the UK will leave the single market and customs union.
With the Conservatives holding only a small majority in the House of Commons, the British prime minister could face defeat in key votes if opposition parties and Conservative rebels reject the government's proposals for a Brexit deal.
Jeremy Corbyn is calling on MPs from all parties to back Labour's stance.
In a speech this morning, Mr Corbyn said: "Labour would seek a final deal that gives full access to European markets and maintains the benefits of the single market and the customs union as the Brexit Secretary, David Davis promised in the House of Commons, with no new impediments to trade and no reduction in rights, standards and protections.
"We have long argued that a customs union is a viable option for the final deal. So Labour would seek to negotiate a new comprehensive UK-EU customs union to ensure that there are no tariffs with Europe and to help avoid any need for a hard border in Northern Ireland."
"New and strong relationship"
Corbyn stressed that the EU would need to ensure the UK has a say in future trade deals if a new customs union is formed.
He also argued: "Labour would negotiate a new and strong relationship with the single market that includes full tariff-free access and a floor under existing rights, standards and protections.
"That new relationship would need to ensure we can deliver our ambitious economic programme, take the essential steps to intervene, upgrade and transform our economy and build an economy for the 21st century that works for the many, not the few."
Mr Corbyn has been a critic of the European Union's "democratic deficit", and during the Brexit campaign called for reform of the union rather than the UK's exit.
However, in his speech today he stressed: "Scepticism is healthy especially when dealing with politicians or the received wisdom of the political and media establishment.
"I am not anti-European at all, I want to see close and progressive cooperation with the whole of Europe after Brexit."
Several Conservative MPs have already backed an amendment that would keep Britain in a customs union with Europe - a move that is likely to pose a major test for Theresa May when it comes to a vote in the next few months.