LONDON — Britain’s opposition Labour Party warned on Sunday that it could team up with Conservative rebels to force the country to become part of an EU customs union, a move that could threaten Theresa May’s government.

Labour’s Brexit spokesman Keir Starmer warned May that “crunch time is coming” over her Brexit plans, which she will set out on Friday, although she has always said Britain would not remain a member of the EU customs union.

The prime minister only holds power thanks to the support of Northern Ireland’s DUP, and there is a serious threat her government could be brought down if a handful of her Conservative MPs team up with opposition parties to demand that she pursues some kind of post-Brexit customs union.

EU supporters back a form of union, which would align tariffs charged on goods imported from the rest of the world, saying it was essential in retaining tariff-free access to the continental market.

“The customs arrangements at the moment are hardwired into the membership treaty so I think everybody now recognizes there’s going to have to be a new treaty — it will do the work of the customs union,” Starmer told BBC television.

“Obviously it’s the only way of realistically to get tariff free access, it’s really important for our manufacturing base.”

He suggested that Labour could back an amendment by pro-EU Tory MP Anna Soubry to a trade bill currently going through parliament, demanding May “take all necessary steps” to secure a trade agreement that “enables the UK to participate after exit day in a customs union with the EU”.

Brexiteers argue that such a deal would prevent Britain from striking free trade deals with the rest of the world, one of the key benefits of leaving the EU.

May is under pressure to set out more details of her position before talks get underway on the future partnership in April.

“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,” said Starmer.

Labour leader Jeremy Corbyn is due to outline the party’s position on Monday, and is expected to announce moves towards a “softer” Brexit, a move that is likely to cause division among the party’s working-class base, the majority of which voted to leave the EU.

Trade minister Liam Fox, an arch euroskeptic, later told the same BBC program that Britain had to be able to set its own trade policy with the rest of the world after leaving the EU in March 2019.

“The key thing about a customs union is that it puts a big frontier around Europe,” he said.

“I want the United Kingdom to make its own decisions. We want to be rule-makers in our own country and not rule-takers.”

May this week brought together her key ministers to thrash out Britain’s negotiating stance.

One minister, who was not present during the Cabinet talks, said they had agreed that Britain would seek to align itself with European Union rules in certain sectors of the economy, but will retain the right to diverge.

EU President Donald Tusk dismissed the reported plans as “based on pure illusion”. — AFP