Updated
The UK government demanded on Wednesday that the EU renegotiate post-Brexit trading arrangements for Northern Ireland after rioting and business disruption hit the province, but the EU immediately rejected the offer.
London stopped short of suspending the so-called Northern Ireland Protocol - introduced at the start of the year, and which requires checks on goods arriving from mainland Britain - and instead called for "significant changes".
It wants the European Union to indefinitely abandon ad-hoc grace periods for certain border checks and freeze legal action launched against the UK for non-compliance, as part of a "standstill period" allowing for fresh negotiations.
Northern Ireland Secretary Brandon Lewis said while Britain had negotiated the protocol "in good faith", its real-world application by the bloc had entailed "considerable and continuing burdens".
"Put simply, we cannot go on as we are," he told UK lawmakers.
"We urge the EU to look at it with fresh eyes and work with us to seize this opportunity and put our relations on a better footing."
But within hours, the EU - which has long insisted that it is up to London to implement what it agreed in their drawn-out Brexit divorce - had ruled out any renegotiation.
"We are ready to continue to seek creative solutions, within the framework of the Protocol, in the interest of all communities in Northern Ireland," European Commission vice president Maros Sefcovic said in a statement.
"However, we will not agree to a renegotiation of the Protocol."
'Jointly agreed'
The protocol was painstakingly negotiated to avoid a hard border with Ireland, by effectively keeping Northern Ireland in the EU's single market.
Northern Ireland, which suffered three decades of sectarian conflict until a peace agreement in 1998, has been rocked by violence this year, in part against the protocol.
Many pro-UK unionists see it as creating a de facto border in the Irish Sea with mainland Britain and say they feel betrayed.
In its proposals, Britain urged the EU to stop broad checks and focus more squarely on goods "genuinely" at risk of entering its single market via Northern Ireland.
The government insisted for all other goods, a light touch was needed to preserve Northern Ireland's integral status as part of the UK.
It also wants the removal of any oversight role by the European Court of Justice.
But the demand to return to the negotiating table will not be welcomed by the bloc, just months after years of gruelling Brexit talks ended.
"The Protocol was jointly agreed by this UK Government and the European Union," Ireland's foreign minister Simon Coveney said.
"It is jointly owned by them, and must be jointly implemented by them," he added, noting Dublin would encourage "realistic solutions in a spirit of positive and constructive engagement".
US 'deeply committed'
Frustrated by the new red tape since the UK left the EU fully at the start of this year, several UK companies have already suspended sales to Northern Ireland, or are offering a reduced choice.
Retail chain Marks and Spencer said that in the protocol's current guise, there will be "gaps on the shelves" in Northern Ireland this Christmas.
But the EU, seeking to preserve the integrity of its single market, says Britain has been acting in bad faith, knowing full well what it signed up to.
The protracted rows over the protocol are drawing concern further afield, with US President Joe Biden following closely.
The US State Department urged both sides to negotiate "within the existing mechanisms".
John Kerry, Biden's climate envoy, said the president and his colleagues were "deeply committed" to making the Good Friday agreement hold and ensuring there was peace.