Sir Keir Starmer has refused to rule out rejoining the European Union in his lifetime as the Prime Minister prepares for crunch talks with continental countries "about how we can be closer".
Sir Keir, who campaigned for a second referendum as Jeremy Corbyn's Shadow Brexit Secretary, declined to comment on his stance on the bloc five times during an interview with The Observer.
The Prime Minister's comments come just 18 months after he ruled out rejoining the EU ahead of the 2024 General Election.
Sir Keir is also coming under pressure from top Labour figures to forge closer ties with Brussels, including from his Deputy Prime Minister David Lammy.
Speaking to The Observer, the Prime Minister instead said: "We've stopped having the discussion about whether we should go backwards and pick over Brexit."
Sir Keir, who has been ramping up his attacks on the architects of Brexit in recent months, also insisted that "the battles coming up in politics" are bigger than him or the Labour Party.
He claimed he is not worried about the prospect of Kemi Badenoch becoming the next Prime Minister.
However, Sir Keir admitted his fears about Nigel Farage winning the next general election.
The Prime Minister refused to rule out Britain rejoining the Brussels bloc five
|PA
"I worry about what will happen to our country in terms of tearing our communities apart," the Prime Minister said.
"Once you say that diverse communities aren't British, that reasonable, compassionate people aren't part of who we are, and that we don't want to look after each other, and that only points of division count, you're going in a different direction."
However, recent opinion polls suggest Mr Farage could succeed Sir Keir in No10.
Find Out Now handed Reform UK an 11-point lead over Mrs Badenoch's Tories, with Labour slumping to a record-low of just 14 per cent.
LATEST DEVELOPMENTS
Pro-Brexit campaigners celebrate Brexit day | GETTYZack Polanski's Green Party leapfrogged Labour into third place, sparking fears among MPs of Sir Keir being outflanked on his left.
Taking the fight to Mr Polanski, Sir Keir told The Observer: "They're anti-Nato at a time when the world is more volatile than it has ever been.
"The Green Party thinks it's all right to sell drugs and there should be no restrictions.
"So, somebody could sell drugs outside my children's school but, if you're a landlord, it should be unlawful. That's nuts. They're dangerous."
Zack Polanski blasted Labour's proposed reforms | PAHowever, the Prime Minister is now plotting to find a way for ex-Deputy Prime Minister Angela Rayner to return to Cabinet in a potential boost to his left-wing credentials.
Ms Rayner, who is reportedly being eyed up by Wes Streeting's allies to stand on a "joint ticket" against Sir Keir, resigned from her role after failing to pay £40,000 in stamp duty on her second home in Hove.
When asked if he misses Ms Rayner, the Prime Minister said: "Yes, of course I do.
"I was really sad that we lost her. As I said to her at the time, she's going to be a major voice in the Labour movement."
Sir Keir also insisted Ms Rayner would return to his Cabinet.
Angela Rayner resigned from the Cabinet earlier this year | PAHowever, Sir Keir's refusal to speak about rejoining the EU comes after the Prime Minister rebuked Mr Lammy over his Brexit comments.
An official No10 spokesman insisted more than 10 times that remaining outside the customs union remains a "red line" for the Prime Minister.
However, Sir Keir continued with his attack against the architects of Brexit in a keynote speech earlier this week.
He said it would be “utterly reckless” to use Brexit as “a template for our future foreign policy” and pledged a “closer relationship” with the EU by the end of Labour’s term in Government.