French President Emmanuel Macron has said the door was "always open" for Britain to remain in the EU despite Prime Minister Theresa May saying Brexit talks would begin next week.

The meeting in Paris followed the leaders' remarkably different election fortunes in the past week.

President Macron's party is headed for a massive parliamentary majority, while Mrs May lost her slim advantage in Westminster.

"Of course the door is always open as long as the negotiations on Brexit have not finished," President Macron said in a press conference.

But he stressed that the British people had taken the sovereign decision to leave the 28-member bloc in their referendum a year ago, adding that the beginning of talks would be a milestone.

"Once it (the Brexit process) has started we need to be collectively clear that it's more difficult to reverse course," he said at the Elysee palace.

Mrs May stressed that she would stick to her timetable of starting Brexit discussions next week in Brussels, saying the talks were "on course", despite her domestic difficulties.

Her Conservative party lost its majority in a bungled snap election last week which some observers suggested might lead Mrs May to abandon her plans for a so-called "hard Brexit".

But she countered: "There's a unity of purpose among people in the UK. It's a unity of purpose having voted to leave the EU that their government gets on with that and makes a success of it."