Emily Thornberry has called on Labour activists to do “whatever it takes” to keep Britain in the EU ahead of a crunch vote on the party’s Brexit policy. 

On Sunday, Labour’s ruling body said the party should only decide how it would campaign in a second Brexit referendum after it had been elected into government – a move reportedly backed by Jeremy Corbyn. 

But just hours before Labour members were due to vote on the controversial policy at the party’s conference in Brighton on Monday, the shadow foreign secretary urged delegates to reject the plan and instead back a motion calling for a pro-Remain stance. 

“With your endorsement today, conference, with the instructions that I hope you give us today, I believe we must strive day and night, whatever it takes, to keep Britain in the European Union,” Thornberry said, having already told the conference she would campaign for Remain. 

It comes a day after the Labour frontbencher revealed to HuffPost UK she had told Corbyn the party must decide “now” whether to back Leave or Remain – or face losing 30% of its core vote to the Lib Dems and the Greens. 

Under plans backed by Labour’s National Executive Committee, the party would decide how to campaign in a referendum “through a one-day special conference, following the election of a Labour government”. 

But the proposal has split Labour’s frontbench, with Thornberry and deputy leader Tom Watson arguing that the party should back Remain in all circumstances. 

Meanwhile, Labour’s health secretary Jonathan Ashworth told a fringe event on Monday that he would also campaign to Remain, because “it’s in the interests of the country, and it’s certainly in the interests of our National Health Service.”

However, he acknowledged that “huge swaths” of the country that normally vote Labour also backed Leave. 

“So I understand why we as a party are trying to offer a proposal that brings the country together, both sides of the country,” Ashworth said.