Sir Keir Starmer has said he will seek a “much better” Brexit deal with the EU if Labour wins the next general election.

The opposition leader told the Financial Times that the current deal, which is due for review in 2025, is “too thin”.

Sir Keir was speaking at a conference of centre-left leaders in Montreal, Canada.

But he ruled out re-joining the customs union, the single market or the EU.

It remains unclear, however, if Brussels would be open to making major changes to the agreement, which was agreed by former Conservative Prime Minister Boris Johnson in 2021.

A Conservative spokesman accused the Labour leader of changing his position, saying: “Three years ago he promised he wouldn’t seek major changes to the UK’s new relationship with the EU, but now his latest short term position is that he will.

“What price would Keir Starmer be prepared to pay to the EU for renegotiating our relationship?”

Sir Keir has repeatedly said he would not seek to rejoin the EU if his party comes to power, promising to “make Brexit work”.

His party has consistently held double-digit leads in the political opinion polls, with a general election expected to take place some time in 2024.

“Almost everyone recognises the deal Johnson struck is not a good deal – it’s far too thin,” he told the Financial Times.

“As we go into 2025 we will attempt to get a much better deal for the UK,” he said, although he did not specify what parts of the deal he would seek to improve.

He added that he was confident a better deal could be negotiated with Brussels, as well as a “closer trading relationship”.

“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,” he said, adding that he was thinking about “future generations”.

“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.”

This is a significant piece of political positioning from Sir Keir. He has spent much of his leadership trying to reassure voters that he would not take Britain back into the EU, or seek membership of the single market or customs union.

That position has not changed. But figures around the Labour leader believe that having got over the message that he does not want to undo Brexit, he has now earned a hearing to talk about changing the terms of the settlement.

The Trade and Co-Operation Agreement signed by Mr Johnson already has a review in 2025 written into it. The current thinking in Brussels is that this would only involve minor tweaks, though Sir Keir may have further-reaching changes in mind, including agreements on deeper trade ties, more exchanges for young people and students and easier rules for touring musicians and artists.

The willingness to put improved relations with the EU at the heart of his political offer is a sign of Sir Keir’s growing political confidence. This was also in evidence last week when Sir Keir visited The Hague, in the Netherlands, to talk about how better co-operation with the EU could help deal with small boats crossing the Channel.

The Conservatives have already seized on Sir Keir’s comments about Brexit. They believe that his position could push Brexiteers who backed the Conservatives in 2019 back towards Rishi Sunak. A Conservative spokesman said that Sir Keir “wants to take Britain back to square one on Brexit, reopening the arguments of the past all over again”.

Sir Keir spent the weekend meeting fellow centre-left leaders in Canada, including the country’s Prime Minister Justin Trudeau.

He is also expected to travel to Paris to meet French President Emmanuel Macron later this week, where post-Brexit relations are expected to feature heavily in talks.

His visit to the Hague last week to meet with the EU’s law enforcement agency Europol, seeking a deal to try and stop smuggling gangs bringing people across the channel in small boats led to accusations by Prime Minister Rishi Sunak and Home Secretary Suella Braverman that his party was planning to let the UK become a “dumping ground” for 100,000 migrants from the continent each year, claims he said were “complete garbage”.

There is some anxiety in Labour about the row Sir Keir found himself embroiled in about whether Labour would accept a quota of asylum seekers from the EU as part of a ‘burden-sharing’ migration agreement. On Sunday, the Labour leader ruled this out after days of debate.