UK Prime Minister Boris Johnson on Saturday warned Europe he would suspend a deal for post-Brexit trading arrangements in Northern Ireland unless a solution could be found to a row over border checks.
Brussels has been angered by London's failure to introduce checks on goods heading across the Irish Sea from the mainland UK -- England, Scotland and Wales -- to the province of Northern Ireland.
European leaders delivered an ultimatum to Johnson on the sidelines of this weekend's G7 summit, urging him to keep his word and implement the Northern Ireland "protocol", which was signed separately from a trade deal between the two sides.
But Johnson defiantly restated his calls for the European Union to compromise, urging a more pragmatic approach after talks on the issue broke down earlier this week.
He said he was not prepared to endanger the territorial integrity of the UK, and would "not hesitate to invoke Article 16" of the protocol, suspending its application.
"We need to sort it out," he told Sky News. "I think we can sort it out. But it's up to our EU friends and partners to understand that we will do whatever it takes and there is some misunderstanding."
Article 16 provides both the UK and EU unilateral powers to take action, if the application of the protocol leads to major social, economic or environmental problems or affects trade.
But there is meant to be an attempt to solve problems first at an oversight committee.
The UK and the European Union signed a last-gasp trade deal in December last year, just weeks before the former's departure from the European single market and customs union.
The protocol is designed to prevent unchecked goods entering the single market via Northern Ireland's border with EU member state Ireland -- the UK's only land border with the bloc.
But London has so far failed to implement full checks due to sensitivities over Northern Ireland, which endured a three-decade conflict over the question of British rule, known as The Troubles.
Pro-British unionists maintain that the protocol drives a wedge between Northern Ireland and the rest of the UK by effectively keeping it in the single market and customs union, and introducing cumbersome checks on arriving goods.
Fears are growing of a repeat of recent violence sparked by the measures, as the so-called "marching season" of hardline unionists begins next month.
London has indicated it will extend a grace period on the delivery of British chilled meat products at the end of this month, prompting EU threats of retaliatory measures, including targeted tariffs.
- EU unity -
Johnson held talks Saturday with French President Emmanuel Macron, German Chancellor Angela Merkel and the heads of the European Commission and the European Council.
All told him directly to abide by the terms of the divorce deal he signed to take the UK out of the EU after nearly 50 years of membership.
"Both sides must implement what we agreed on," said European Commission president Ursula von der Leyen and European Council chief Charles Michel.
"There is complete EU unity on this."
A source in Macron's office said he told Johnson in no uncertain terms there was a need to "reset" relations, and that the UK should "keep its word", effectively dismissing UK calls for flexibility.
The director-general of the World Trade Organization, Ngozi Okonjo-Iweala, said separately she hoped a solution could be found as a UK-EU trade war "is not what the world needs right now".
But Downing Street said afterwards that Johnson had told EU leaders he would not change tack.
The Times newspaper reported this week that President Joe Biden had ordered US diplomats to rebuke London for threatening the peace in Northern Ireland with its stance.
Bill Clinton's administration was a key player in securing a landmark 1998 Good Friday Agreement that ended the Northern Irish conflict.
Biden -- a proud Irish-American with distant relatives still in Ireland -- has taken a keen interest, with warnings it could threaten a hoped-for UK-US trade deal.
Johnson played down any rift after talks with Biden on Thursday, insisting the US president did not sound the alarm over the issue and there was "absolutely common ground" on all sides in keeping the peace.