European Council chief Charles Michel warned Britain Sunday its international credibility is at stake, saying it must fulfil its responsibility to implement the Brexit withdrawal agreement.
Michel made the comment on Twitter after speaking by telephone with Irish prime minister Micheal Martin, as London prepares legislation that could upend a hard-won compromise over the EU member's border with British Northern Ireland.
It is "time for UK Government to take its responsibilities. International credibility of UK signature at stake," Michel wrote.
"Withdrawal Agreement to be fully implemented, ensure peace & stability in Ireland & preserve the integrity of Single Market," he added.
The British government on Wednesday introduced new legislation to rewrite the Brexit withdrawal treaty -- a bill that is causing deep alarm among former prime ministers and Prime Minister Boris Johnson's own MPs.
Under the EU withdrawal treaty, Northern Ireland will enjoy a special status to ensure no return of a border with EU member Ireland, in line with the 1998 peace pact that ended three decades of bloodshed.
But Johnson on Saturday accused the EU of threatening to tear the UK apart by imposing a food "blockade" between Britain and Northern Ireland.
EU negotiator Michel Barnier Sunday said that the protocol on Ireland and Northern Ireland "is not a threat to the integrity of the UK.
"We agreed this delicate compromise" with Johnson and the British government "in order to protect peace & stability on the island of Ireland", he said on Twitter.