Irish police officers patrol before U.S. House Speaker Nancy Pelosi's visit to the border between Ireland and Northern Ireland in Bridgend, Ireland April 18, 2019. REUTERS/Clodagh Kilcoyne - RC11AB65A580

BRUSSELS (Reuters) - Any new British prime minister will still face the problem of what to do about the Irish border when Britain leaves the European Union, the EU’s Brexit negotiator Michel Barnier said on Friday.

Managing the sensitive Irish border and designing an emergency “backstop” solution to prevent the return of extensive controls after Brexit has proven the most contentious element of divorce negotiations between London and the EU.

British Prime Minister Theresa May stepped down as leader of her ruling Conservatives on Friday, officially triggering a contest to replace her that could see her party embrace a tougher stance on Brexit.

“The problem is there (with the Irish border), created by Brexit. We have to protect peace. Stability is not a question of goods or customs. It’s a question about people in Ireland, on both sides,” Barnier told a conference in Slovakia.

“In any case, a new prime minister will not change the problem. The problem is there and the new prime minister will have the responsibility with us to solve this problem.”

While Barnier declined to comment directly on May’s possible successor or comments by some of the candidates who have vowed to renegotiate the Brexit deal May had sealed with the EU but then failed to ratify at home, he did reiterate the bloc’s long-standing position on the stalled divorce.

“The point is to know whether the UK still wants to leave in an orderly manner,” Barnier said. “If this answer is ‘yes’... the Withdrawal Agreement is the only one possible.”

He said any ‘no-deal Brexit’, involving the UK’s departure without a negotiated divorce settlement, would never be the choice of the EU, which is determined to defend its cherished single market.

“What is at stake in Ireland is the peace and people and stability in the first place. Second - the single market, which is our main asset and nobody can ask us to weaken it... We are not ready to compromise, to... bargain on the single market.”