France’s state secretary for European affairs has confirmed that the EU27 are not prepared to reopen the Brexit withdrawal agreement, and that without a “new political line” in the UK or a second referendum, Britain must expect to leave the bloc on 31 October.

On the eve of a two-day working visit to London, Amélie de Montchalin also told the Guardian that France regarded the €39bn financial settlement Britain has agreed to pay the EU as part of the exit deal as a matter of international law.

“We are now waiting for clarification from the UK side,” De Montchalin said. “We consider it is up to Britain to decide how it wants to proceed. The exit agreement was not negotiated against the British; negotiators on both sides tried, painstakingly, to find the best solution for all concerned.”

Several Tory leadership candidates have said that if they are selected, their priority will be to go to Brussels and seek to renegotiate the deal agreed last December – and that if they do not succeed, Britain must be ready to leave without a deal. Others have said a further extension could be necessary.

Boris Johnson, the favourite to succeed Theresa May as party leader and prime minister, has pledged to consider withholding Britain’s Brexit “divorce” payment until the EU27 agree to improve the terms on which the UK’s leaves, describing the money as “a great lubricant” in getting a good deal.

But De Montchalin, who replaces France’s former Europe minister Natalie Loiseau, now an MEP, said the bloc was not willing to reopen the 599-page exit agreement. “If Britain does want to leave, and if it wants to leave in an orderly fashion, then this is the way it must do it,” she said.

She said the only condition on which France would be prepared to grant a further extension to the 31 October deadline would be if there were to be a “profound change” to Britain’s current political stance on Brexit.

“As President Macron has said, if there is a totally new political line in Britain, the Europeans would be prepared to reconsider,” she said. “But for now, 31 October is the final deadline.” A no-deal Brexit was “not what France wants”, de Montchalin added, “but we are prepared for it, and so it is now a realistic option.”

Emmanuel Macron said last week he considered 31 October to be the “final, final deadline” for Britain’s much delayed departure, saying he did not want the new European Commission and executive to have to deal with Brexit.

De Montchalin described the €39bn that Johnson has threatened to hold back as “not a settling of scores, or some kind of exit bill – this sum simply represents Britain’s engagements. So this is now a matter of respecting international law … Britain will always be France’s neighbour.”

She said the EU 27 would remain united through the next Brexit stage, dismissing suggestions that some leaders might be ready to break ranks. Jeremy Hunt, the foreign secretary, claimed this weekend that the German chancellor, Angela Merkel, would be willing to renegotiate the UK’s Brexit deal and possibly the Irish backstop.

“Every single member state realises the importance of maintaining that unity and engaging with the UK through the the EU,” she said. “There can be no mini-deals. It’s a question of the credibility of the bloc. And all have understood the importance of the future relationship with Britain.”

She said it would be “unacceptable” for France or any other EU member state to interfere in the present impasse. “I’m certainly not going to say the UK is wasting its time,” she said. “The problem is that there are almost as many kinds of Brexit as there are MPs. But we do need clarity now. We need a decision.”