Britain on Tuesday said it sent the EU a new legal text to overhaul fractious post-Brexit trading arrangements in Northern Ireland, and renewed threats to abandon the current deal.
The legal proposal, dispatched on the eve of the European Union setting out its own plans, comes amid vehement opposition to the so-called Northern Ireland protocol from some in the British province.
It follows months of demands by London for the pact to be revised to change the rules around internal UK trade and the arbitration mechanisms governing it.
In a speech in Lisbon, Brexit minister David Frost argued the new text worked "with the grain" of the existing deal, but would "deliver significant change" and put it on a "durable footing".
"Without new arrangements in this area, no protocol will ever have the support across Northern Ireland it needs to survive," he said, reiterating warnings London could trigger a clause scrapping the agreement.
"We would not go down this road gratuitously or with any particular pleasure," he said.
"It is our fundamental responsibility to safeguard peace and prosperity in Northern Ireland, and that is why we cannot rest until the situation has been addressed."
Britain voted to leave the EU in a landmark referendum in 2016. When it eventually departed from its rules and regulations at the start of 2021, the Northern Ireland protocol came into effect.
It has kept the British-ruled province inside elements of the EU customs union and single market in order to prevent a hard border with EU member Ireland.
The border was a former flashpoint in the sectarian conflict over British rule between pro-UK unionists and pro-Ireland nationalists, which wound down in 1998.
However, the protocol has required new checkpoints at ports in the region to stop the risk of goods from England, Scotland and Wales getting into the EU by the back door.
Pro-UK unionists in Northern Ireland say it has created a border in the Irish Sea that undermines the province's place in the wider UK, and strengthens pro-Irish republicans' case for a united Ireland.
Frost denied Britain had agreed the protocol in December 2019 in bad faith without intending to implement it, insisting that it took "a risk" and blamed Brussels for its implementation.
His proposed changes include allowing goods to circulate virtually freely between Northern Ireland and the rest of mainland Britain and removing the European Court of Justice (ECJ) as the scheme's ultimate arbiter.
"For the EU now to say that the protocol -- drawn up in extreme haste in a time of great uncertainty -- can never be improved upon, when it is so self-evidently causing such significant problems, would be a historic misjudgement," he warned.
His remarks came as the European Commission was poised to unveil its own plans Wednesday to accommodate British misgivings, with the mood in Brussels increasingly frustrated.
"There is no appetite among the member states to be strung along by Britain," an EU diplomat told AFP on Monday.
"The window for dialogue is closing and it's not good that even before the EU makes its proposal it is refused.
"This proposal will be as far as the EU can take it."