Remain supporters holds up a placards and flags of United Kingdom and the European Union outside The Palace of Westminster to show their support for "Stopping Brexit" in London on Monday. Credit: Alastair Grant | AP

U.K. labor unions voted for a campaign against any Brexit deal reached by Prime Minister Theresa May that fails to address the needs of working people — with a referendum central to their strategy for the fight.

The motion, backed overwhelmingly by the Trades Union Congress’ annual meeting in Manchester on Monday, says the Conservative government is not “capable” of “delivering a Brexit deal that will work for ordinary people.”

The decision will add to pressure on Jeremy Corbyn, leader of the main opposition Labour Party, to back a “people’s vote” on the final deal agreed with Brussels. He’s been trying to straddle the Brexit divide to balance the fact that while most districts the party represents backed leaving the EU in the 2016 referendum, the majority of its supporters nationwide voted to remain.

The party, which holds its own conference in two weeks’ time, has said it respects the 2016 result and that its preference is to win power via a general election so it can “prioritize jobs and living standards” in talks with Brussels.

But the support of unions will quiet claims that the campaign for a second referendum is being used by party members as a vehicle for undermining Corbyn — another reason the leadership has been resistant to back it.

“Our movement must be prepared, politically and industrially, to mobilize,” the motion approved by Trades Union Congress delegates said. Congress “calls for the option of a public vote to be kept on the table.”

The Trades Union Congress, an umbrella group for Britain’s 5.5 million-strong organized labor movement, backed the Labour Party’s call for an early general election, but recognized that May would be unlikely to call one if her Brexit deal fails to pass through Parliament.

After a period of stalemate, there’s been a more optimistic tone from both sides of the U.K.-EU talks in recent days. EU chief Brexit negotiator Michel Barnier said Monday a deal on Britain’s orderly withdrawal from the bloc is possible within eight weeks.

The unions’ motion warned that the prospects of any deal meeting their needs are “remote.”

“If you come back with a deal that doesn’t put workers first, and if you won’t call a general election, then I’m warning you we’ll throw our full weight behind a campaign and demand that the terms of a deal are put to a popular vote,” Trades Union Congress General Secretary Frances O’Grady said in comments directed at May during a speech. “When it comes to our future, one way or another, people must have a say.”

Len McCluskey, leader of Unite, Britain’s biggest union, and an enthusiastic supporter of Corbyn, said “a people’s vote on the deal” should be kept on the table “as a safety net if Tory MPs are too spineless” to block a bad deal.

“The vote that we desperately need above all is a general election,” he said.

There were dissenting voices, as some speakers warned that another vote would add further to the division in British society over Brexit. Mick Cash, of the RMT union, described a people’s vote on the deal as “nothing more than a Trojan Horse for a second EU referendum.”

Dave Ward, general secretary of the Communication Workers Union, warned a second referendum “would be the biggest mistake our movement could make” as it would split communities when unity is needed to pursue a change of government.

But he and his union backed the motion as he, too, said a people’s vote should not be ruled out. “That may come and that may have its moment,” he said.

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