German Chancellor Olaf Scholz on Tuesday opposed any challenge to post-Brexit controls establishing a special status for Northern Ireland, which London says are "not tenable" after the Sinn Fein republicans won recent elections in the UK territory.
The UK has hinted at the possibility of unilaterally suspending part of the Brexit agreement if no new deal can be reached with the EU.
"No one should unilaterally cancel, break or in any way attack the settlement we have agreed together, especially because we know that this is a complex issue which is not only about the relationship between the European Union and the United Kingdom, but also has to do with the peaceful development of Ireland," Scholz said.
Belgian Prime Minister Alexander De Croo, appearing alongside him at a joint news conference in Berlin, also said he was opposed to such a move.
"It has always been important that we find a comprehensive solution that meets all these requirements at the same time, and that is what we are committed to, without change," the German Chancellor insisted.
The Sinn Fein republicans, supporters of Irish reunification, won local elections for the first time on Thursday.
This puts them in a position to run the local executive, which is supposed to be shared with unionists committed to remaining part of the British crown under the 1998 peace agreement.
But paralysis threatens: for the Democratic Unionist Party, now the second largest force in the local assembly, there is no question of participating in a government unless the special post-Brexit status is changed.
The Northern Ireland Protocol, which sets out the rules, is "not tenable in its current form" given the elections in the British province, Prime Minister Boris Johnson warned earlier on Tuesday morning.
The DUP vehemently opposes the protocol, which creates a trade barrier with the rest of the UK in the form of customs and regulatory checks at Northern Irish ports.
A spokesman for Boris Johnson said no decisions on next steps had been taken yet by the British government.
"We will continue to talk with the EU but we will not let that stand in the way of protecting peace and stability in Northern Ireland," said a British briefing document published on Tuesday.
Irish counterpart Micheal Martin also said he had warned Johnson against taking unilateral action. Brussels is looking for changes within the terms of the existing deal.
The agreement between London and the EU, part of the Brexit divorce treaty, keeps Northern Ireland subject to some EU rules and inside the EU single market for goods in order to preserve an open border with EU member the Republic of Ireland to the south.
London has been trying for months to renegotiate the protocol with Brussels, without any significant progress so far, and has threatened on several occasions to suspend unilaterally certain provisions in the absence of an agreement.
Brussels put forward detailed proposals last October designed to ease the impact of the new arrangements. This year it announced significant changes to post-Brexit regulations to allow medicines to continue to flow between Britain and Northern Ireland.
Although the protocol came into force in January 2021, grace periods on agri-food controls between Britain and Northern Ireland were imposed unilaterally by the British government to allow businesses to adapt to new arrangements, and were later prolonged indefinitely.
The Northern Ireland Protocol was not imposed on the United Kingdom by Brussels, but was negotiated by Boris Johnson himself and his government in the autumn of 2019, several months after he became prime minister.
The difficulties surrounding the Irish border were well known, the issue having been one of the root causes of the parliamentary deadlock that paralysed former prime minister Theresa May’s government and led to repeated delays in the UK’s departure from the EU.