LONDON — Labour leader Jeremy Corbyn made a major intervention in the campaign against Brexit Thursday by pledging to scrap the EU’s controversial free trade deal with the United States.

In a speech in central London, Corbyn promised to strike down the Transatlantic Trade and Investment Partnership — known as TTIP — if he is elected prime minister before it comes into force. He also vowed to work with rebel Tories and Scottish nationalists in parliament to block the deal.

TTIP has become a major issue for left-wing activists over claims it could undermine the government-funded National Health Service by forcing public hospitals to open up contracts to private U.S. healthcare firms.

However, Corbyn’s intervention is likely to spark fury among Remain supporters, who have made the EU’s ability to a strike a trade deal with Washington a central message of their campaign.

Corbyn will provoke further concern among Labour MPs by refusing to take part in a prime-time live TV debate on the BBC two days before the referendum later this month.

The Labour leader is adamant he will not share a platform with any member of the Conservative Party.

Instead Corbyn’s team are pushing for Angela Eagle, Labour’s shadow business secretary, to take his place at the event, scheduled for June 21 at Wembley Arena, London, although the Remain campaign are understood to favor the leader of the party’s pro-EU campaign, former home secretary Alan Johnson.

David Cameron has also refused to take part in the event, to avoid a “blue on blue” confrontation with other leading Conservatives lining up for the Out campaign.

Corbyn’s full-frontal attack on TTIP Thursday will be jumped on by the Leave campaign, which was badly burned by Barack Obama’s dramatic declaration during a visit to London in April that the U.K. would have to go to the “back of the queue” in future trade negotiations if it left the EU.

The Remain campaign claimed Obama’s intervention was proof that the U.K. economy would be worse off outside the EU.

Labour MPs will be concerned that Corbyn’s threat to scupper the trade agreement undermines this argument.

A senior Labour source dismissed claims Corbyn’s intervention would damage the Remain campaign — insisting it would reassure left-wing party members.

“Labour MPs will have to accept it,” the source said. “We are going to be committed against TTIP. Our supporters need to be reassured on that.”

While Corbyn’s position is consistent with his long-held opposition to trade liberalization, it has already caused consternation among Labour MPs who believe it simply undermines support for the EU.

Corbyn, however, believes the move is necessary to shore up working-class support for remaining in the EU, amid growing anger over globalization, multinational firms and free movement.

Thousands of Labour activists had written to him voicing their opposition to TTIP, he said, adding that they were “concerned, rightly, that it could open up public services to further privatization” and water down social protections.

He said he backed French President Francois Hollande’s pledge to block the current deal, adding: “Today we give this pledge, as it stands, we too would reject TTIP — and veto it in government.”

Tory warning

The Labour leader also used Thursday’s speech at the Institute of Engineering Technology to open up a new line of attack against Brexit, by warning Labour supporters not to allow pro-Brexit Tories to manage the U.K.’s withdrawal from the EU.

“A vote to Leave means a Conservative Government would then be in charge of negotiating Britain’s exit,” Corbyn said. “Everything they have done as a government so far means we could not rely on them to protect the workplace rights that millions rely on.”

Corbyn praised the EU’s social chapter, guaranteeing paid leave, and the working time directive, limiting the number of hours people have to work.

“It’s important to understand the benefit of these gains,” he said. “It means workers throughout Europe have decent rights at work, meaning it’s harder to undercut terms and conditions across Europe.

“Several Leave supporters have stated clearly they want to leave Europe to water down workers’ rights, to rip up the protections that protect work-life balance, that prevent discrimination and prevent exploitation and injustice.

“That is why we say, the threat to the British people is not the European Union — it is a Conservative government here in Britain, seeking to undermine the good things we have achieved in Europe and resisting changes that would benefit the ordinary people of Britain.”

But the Labour leader insisted that the EU needs to focus on bringing in more social protections rather than pursuing free trade deals.

“When we make the case to remain, we also make the Labour case for reform,” he said.

“We can reform to get a better deal for consumers; to strengthen workers’ rights across Europe and prevent the undercutting of wages; to meet the challenges posed by migration and the refugee crisis; to end the pressure to privatize public services; to democratize the EU’s institutions and bring them closer to people; and for reforms to ensure we generate prosperity across Europe to the benefit of all.”

Support among European leaders for TTIP has been faltering, with Hollande saying that his country would say “no” to TTIP at the negotiations’ current level, and German Vice Chancellor Sigmar Gabriel saying his Social Democrats don’t “wish to be part of a bad deal.”

European Commission President Jean-Claude Juncker said on Monday that he will ask EU leaders to reconfirm their commitment to TTIP at a European Council in late June.

However, European Trade Commissioner Cecilia Malmström said this week that even if there is no agreement this year, that doesn’t mean the talks have failed. Not securing a deal before Barack Obama leaves office, which was the ideal scenario for TTIP negotiators, “doesn’t mean the death of TTIP; it means delay of TTIP,” Malmström said.

TOM MCTAGUE