European Council chief Charles Michel has warned the UK its international credibility is at stake, saying it must fulfil its responsibility to implement the Brexit withdrawal agreement.
Mr Michel made the comment on Twitter after speaking by phone with Taoiseach Micheál Martin.
It is "time for UK government to take its responsibilities. International credibility of UK signature at stake," Mr 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 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 Ireland, in line with the Good Friday Agreement.
But yesterday, Mr Johnson accused the EU of threatening to tear the UK apart by imposing a food "blockade" between Britain and Northern Ireland.
The EU's chief Brexit negotiator Michel Barnier said today 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.
Former prime ministers John Major and Tony Blair have united to urge MPs to reject Mr Johnson's legislation, while Labour leader Keir Starmer accused Mr Johnson of having "reigniting old rows" by working to flout his own Withdrawal Agreement, but pledged Labour support if he addresses "substantial" concerns.
Tory rebels suggested their numbers were growing and opinions were only hardened by Mr Johnson's increased rhetoric that the EU could impose a trade border in the Irish Sea.
Mr Johnson spoke with around 250 MPs on Friday to try to drum up support for the bill, and warned them against a return to the "miserable, squabbling days of last autumn" over Brexit.
In an article for The Telegraph, Mr Johnson said the EU was threatening to use an "extreme interpretation" of the Northern Ireland Protocol to impose "a full-scale trade border down the Irish Sea" that could stop the transport of food from Britain to Northern Ireland.
Mr Major and Mr Blair, the former Conservative and Labour leaders, have united to condemn Mr Johnson's controversial UK Internal Market Bill in an article for the Sunday Times.
"We both opposed Brexit. We both accept it is now happening. But this way of negotiating, with reason cast aside in pursuit of ideology and cavalier bombast posing as serious diplomacy, is irresponsible, wrong in principle and dangerous in practice," they said.
"It raises questions that go far beyond the impact on Ireland, the peace process and negotiations for a trade deal - crucial though they are. It questions the very integrity of our nation."
Mr Starmer called on Mr Johnson to throw out clauses that could breach international law and those that lead the devolved administrations to warn of a "power grab" to get Labour support.
"If the Government fixes the substantial cross-party concerns that have been raised about the Internal Market Bill, then we are prepared to back it," he wrote in the Sunday Telegraph.
"But if they do not, and the talk collapse, then it is their failure and incompetence that will have let the British people down."
Minister for Justice Helen McEntee said she would not like to see the UK implementing a bill that could potentially breach international law and damage trust between the UK and Ireland in the Brexit process.
Speaking on Sky News, Minister McEntee called for the UK government to redact the measures in the Internal Markets Bill to ensure that trust is not breached.
She said Northern Ireland First Minister Arlene Foster accepted the Northern Ireland protocol was "an international agreement and something that had to be implemented" and any suggestion it is going to create any new border is simply not true.
Minister McEntee said the protocol ensures there are mechanisms in place to make sure there is no unfair advantage for the UK post-Brexit.
DUP will attempt to amend Internal Market Bill
The DUP will attempt to amend the Internal Market Bill to ensure the UK sets the rules on state aid in Northern Ireland.
The party's Westminster chief whip Sammy Wilson criticised Mr Major and Mr Blair after they warned the UK's latest proposals could undermine the Good Friday Agreement.
He said: "The Internal Market Bill is far from perfect and that's why we will table amendments to the Bill.
"The suggestion by the hero of the peace process brigade that the Bill rips apart the Belfast Agreement is complete and utter bunkum without any factual basis.
"They need to explain how making it easier for Northern Ireland to do business with our biggest market undermines the Belfast Agreement.
"They need to explain how Northern Ireland companies having less paperwork undermines the Belfast Agreement.
"They need to explain how our amendment which would ensure the UK sets the rules on state aid in Northern Ireland undermines the Belfast Agreement.
"Such a tool is vital to help us fend off predatory behaviour from our nearest competitor."
The East Antrim MP said the Belfast Agreement contained one substantive mention about the Irish border and that related to army demilitarisation - which has already happened.
He added: "The Belfast Agreement however is about respecting consensus, something which the Withdrawal Agreement ignored and drives a coach and horses through.
"Blair and Major were less outspoken about the impact of a trade border between Northern Ireland and the rest of the UK despite its obvious impact on our economy and relationships in Northern Ireland.
"The Internal Market Bill as published is not the finished product but it is a massive step forward for business in Northern Ireland."
Additional reporting AFP