The stalled EU-UK divorce treaty will not change with the arrival of a new prime minister in London, the outgoing head of the European Union's executive said on Tuesday.

European Commission President Jean-Claude Juncker reiterated the EU's refusal to renegotiate as many of the Conservative Party candidates vying to replace Prime Minister Theresa May said they would seek a new agreement.

"I have the impression for months now that the interest for the British political society is how to replace PM May, not how to find an agreement with the EU," Juncker said seven months after May sealed a deal with the bloc but then failed to get it ratified by parliament at home.

"It's not a treaty between Theresa May and Juncker, it's a treaty between the EU and the U.K. It has to be respected by whomsoever will be the next prime minister ... There will be no renegotiation."

He rejected putting an expiry date on the backstop - an insurance policy for the Irish border that is a key point of contention in the divorce process.

Earlier, the European Commission was asked to comment on a pledge from Boris Johnson, the favorite to succeed May, to withhold billions in liabilities owed to the EU budget.

"Everybody knows what is on the table. What is on the table has been approved by all member states and the election of a new prime minister will not change the parameters," said spokesman Margaritis Schinas.

Asked again about Johnson at an event in Brussels, Juncker reiterated that the deal cannot be changed and refused to speculate on who could replace May.

"It's the decision of the Conservative Party to put a new prime minister, we have to deal with the new prime minister ... it's not our choice and nobody asked our opinion."

"My working assumption is that the British will leave before October 31st, 2019," said Juncker, who is stepping down as the head of the Commission on that day.