Tony Blair is said to have set up the meeting between Sir Keir Starmer and Emmanuel Macron, as part of a covert plan to rejoin in the EU.
The former Prime Minister planned the meeting to coincide with the announcement of EU 'associate membership', a scheme which France is said to be keen for the UK to be part of.
An insider said Blair is "convinced that Brexit is now a vote-winner", pushing Starmer to open the door to its reversal in order to win the election.
Starmer met with Macron in Paris yesterday. The duo are understood to have spoken for 45 minutes without aides present.
Starmer described the meeting as a 'very political discussion'
PA
Speaking about the meeting, the Labour leader said: "It was my first opportunity to say how much I value the relationship between our two countries, particularly when it comes to prosperity and security, and how, if we are privileged enough to be elected into power, I intend to build on that relationship and make it even stronger than it is today.
He added: "We had a very political discussion covering a lot of issues to do with global politics.”
The meeting took place yesterday, the same day as the EU's plans for associate membership of the bloc were revealed.
The plans are said to be aimed at a Labour Government and would not require the UK to undertake another EU referendum, as it would stop short of full membership.
The EU hopes to enact the plans by 2030.
A European diplomatic source said: "It is carefully balanced politically to be a potential place for Britain without the need to ever rejoin the EU or to hold a referendum."
While the plans for associate membership do not go as far as a customs union, they do include legal integration.
Under the proposals, the UK would have speaking rights among the councils of ministers discussing the bloc's legislation, but it would not have a vote.
Speaking about Blair's role in Starmer's trip to France, the source told the Daily Mail: "'Blair is trying to convince Starmer that the more he edges towards saying Brexit is a disaster, the better Labour will do.
"He believes that reversing Brexit can be a vote-winner and he is pushing that message very hard."
This week, Starmer revealed that he plans to renegotiate the UK's deal with the EU in 2025, in order to strike up a new relationship with the bloc.
The Labour Leader said he wants a closer trading relationship with the bloc, dismissing the 2020 deal struck by Boris Johnson as "not a good deal".
But insiders have claimed the UK would have to rejoin the single market in order to secure a better deal.
Jacob Rees-Mogg told GB News that Starmer's planned renegotiation of Brexit would be the equivalent of a second referendum on Brexit but this time, done "by stealth".
He explained: "As a man who has always been a starry-eyed Europhile, who of course was a champion of the second referendum. Why do you champion a second referendum? Because you believe the voters got it wrong the first time and need to be given a second choice. Now he wants to do it by stealth.
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"Because renegotiation leads to dynamic alignment. Essentially, we would shadow the EU without becoming a member. So we would be like Norway - a rule taker without the democratic accountability that we now have for our laws by being an independent nation-state. So it would restore the democratic deficit."
Meanwhile, a Conservative Party insider told GB News that Labour would happily "creep back to EU", even if it meant heading back into the Euro.
A source close to the eurosceptic European Research Group added: "I think hardly any surprise in regard to where his heart really lies.
"He will always remain a Remainer and so I think hardly anyone is that shocked once his true colours start emerging."