The Labour leader spent the weekend meeting fellow centre-left leaders in Montreal, Canada, including the country's prime minister Justin Trudeau.

The party has long been critical of the deal negotiated by the Conservatives under Boris Johnson, with the Trade and Cooperation Agreement up for review in 2025.

Speaking to The Financial Times, Sir Keir said: "Almost everyone recognises the deal Johnson struck is not a good deal - it's far too thin.

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"As we go into 2025 we will attempt to get a much better deal for the UK."

Rishi Sunak's spokesman said the government was not planning to "relitigate the past or reopen it in any way, shape or form," adding: "Obviously there is a set statutory review period but beyond that we're very much focused on maximising the opportunities it presents for the public."

Labour has ruled out rejoining the customs union or single market, but Sir Keir said he was confident a better deal could be secured from Brussels.

"I do think we can have a closer trading relationship as well. That's subject to further discussion."

"We have to make it work. That's not a question of going back in. But I refuse to accept that we can't make it work. I think about those future generations when I say that.

"I say that as a dad. I've got a 15-year-old boy and a 12-year-old girl. I'm not going to let them grow up in a world where all I've got to say to them about their future is, it's going to be worse than it might otherwise have been.

"I've got an utter determination to make this work."

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Sky News's Sam Coates said: "What's interesting about this is that Keir Starmer is on the front foot about something that has been a difficult issue for Labour…

"Until now the Labour leader has been quite cautious about a closer relationship with Europe for fear of alienating those who voted for Brexit in 2016 and then Tory rather than Labour in 2019."

Meanwhile, Labour is also facing renewed pressure from its main union backer Unite, which is reportedly set to launch a grassroots campaign to call for more radical policies on energy and steel.

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Unite general secretary Sharon Graham has called for public ownership of energy and has been critical of Sir Keir in the past.

She told The Guardian the union would be targeting so-called "red wall" areas as part of a bid to put pressure on the Labour leadership.