The French President became the latest in a string of key EU figures to make clear Britain could still choose to step back from a decision widely seen as economically ruinous on the continent.
“If tomorrow, or the day after, the United Kingdom decided to change its mind, it’s clear that we would look at this with kindness,” said a key aide, speaking on behalf of the French head of state.
Briefing reporters in Paris, the source added: “But it’s not up to us if the United Kingdom wants to change its mind.”
The warm words – issued just hours before Mr Macron meets Mrs May at the Sandhurst military academy – go further than his comments at the two leaders’ last bilateral meeting, last June.
On that occasion, he said “the door remains open” to the UK staying in the EU, but acknowledged it would become “more difficult to move backwards”, the further the negotiations progressed.
The French President’s enthusiasm for Britain to think again comes after the European Council President, Donald Tusk, said the country could still “change its mind” - telling the British people “our hearts are open to you”.
“We would be prepared to discuss it. We are not throwing out the British, we want them to stay,” Mr Tusk argued. “And if they want to, they should be able to.”
His counterpart at the Commission, Jean-Claude Juncker, went further by arguing the UK could also opt to rejoin if Brexit went ahead, using the little-known treaty clause Article 49.
Mr Macron is likely to repeat his message when he stages a joint press conference with the Prime Minister, at the close of the summit, when they will issue a communique.
It is also expected to include details of Mrs May’s agreement to accept more refugees, including unaccompanied children, stranded in Calais, as part of a new treaty.
France said the new agreement would include “precise commitments” from the UK to speed up the processing of asylum claims and accept more people attempting to reach the UK.
Britain has already announced it will spend £44.5m to boost security at the French port – widely seen as the price for border controls to remain on the French side of the channel.
Announcing the £44.5m allocation, the Home Office said it would help pay for security fencing, CCTV cameras and "detection technology" in Calais and at other ports along the Channel.
It would build on previous security work in the area, which had helped reduce illegal attempts to enter the UK from more than 80,000 recorded attempts in 2015 to just over 30,000 in 2017.
British participation in Mr Macron’s planned European Intervention Initiative (EII), to enhance coordination between the continent’s armies, will also be confirmed.
And the UK will send three RAF Chinook helicopters to Mali, to bolster the French counter-terrorism operation in the African country, but purely in a logistics role.
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