The Labour leader is set to hold talks with French president Emmanuel Macron in Paris on Tuesday and meet business leaders. He wants to build a “closer trading relationship” with the European Union and strengthen the Brexit deal struck by Boris Johnson which he branded “far too thin”.
But experts stressed that without rejoining the single market or customs union, which Labour has ruled out, Sir Keir would have limited room to manoeuvre to reshape trade relations with Brussels if he became prime minister. Axing trade barriers with the European bloc, when the Trade and Cooperation Agreement is reviewed in 2025, may only be possible if Britain, to some degree, becomes more of a rule-follower, having been a joint rule-maker when in the EU.
On new Brexit arrangements, Quentin Peel, associate fellow on the Europe Programme at think tank Chatham House, said: “It’s by no means straightforward. The closer you are going to be, the more you will have to follow EU rules.”
On trade, he believes “more bits and pieces” could be improved such as veterinary regulations, while major progress could be made on better co-ordinating security and foreign policy, and getting access to the Erasmus scheme to study abroad.
John Springford, deputy director of the Centre for European Reform think tank, added: “‘Some bolt-ons to the EU-UK trade deal would help some sectors, like agriculture.
“But they wouldn’t change the problem: a free trade agreement is much less effective than a single market and customs union. Starmer says he wants a closer EU relationship to improve growth, but his red lines on the EU make it very hard to achieve that.”
Downing Street has said it will not seek to renegotiate the post-Brexit trade agreement with the European Union after Labour pledged to seek a "much better deal".
Asked whether the Government thinks the deal should be renegotiated in 2025, the Prime Minister's official spokesman said: "No, we expect the TCA (trade and co-operation agreement) to remain the basis of our relationship with the EU and are focused on maximising the opportunities it presents us with."
Asked if it can be improved, he said: "We are focused, as I say, on taking the TCA and using our Brexit freedoms to the benefit of the public already.
"We're not looking to relitigate the past or reopen it in any way, shape or form.
"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."
The Labour leader spent the weekend meeting fellow centre-left leaders in Montreal, Canada, including the country’s prime minister Justin Trudeau.
Meeting Mr Macron on Tuesday will be seen as another sign that world leaders believe Sir Keir may be Britain’s next prime minister.
Speaking to the Financial Times ahead of his French trip, Sir Keir said: “Almost everyone recognises the deal Johnson struck is not a good deal — it’s far too thin. We will attempt to get a much better deal for the UK.”
He added: “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’.”
But the Tories accused the Labour leader of flip-flopping on whether he would seek a major shake-up of the Brexit deal. Pensions minister Laura Trott said Sir Keir had changed his position on Brexit multiple times, including previously pushing for a second referendum.
“I think the question is, what price is he willing to pay to reopen that trade deal?
“And it’s something that I’m very concerned about,” she added.
The Bank of England has said Brexit has reduced the capacity of Britain’s economy to grow, weighing on investment and productivity.
Earlier this year, the BoE’s deputy governor Ben Broadbent said the impact of leaving the EU had fed through into the economy faster than the central bank had expected, although the effects had not been larger than anticipated.