Boris Johnson said on Saturday that Britain will do "whatever it takes" to protect its territorial integrity, in the latest escalation of a spat involving the Northern Ireland protocol.
As talks continued at the G7 summit in Cornwall this weekend, the prime minister told Sky News that the EU "misunderstand that the UK is a single country and a single territory".
The Northern Ireland protocol has been a focus of contention in Brexit negotiations since the end of the transition period in December 2020.
"I think we can sort it out but... it is up to our EU friends and partners to understand that we will do whatever it takes," Johnson said.
"I think if the protocol continues to be applied in this way, then we will obviously not hesitate to invoke Article 16," he added.
Article 16 is a safeguard clause which allows either side to take measures if they feel the agreement has led to economic, societal or environment difficulties.
Johnson claimed that the protocol is having a "damaging impact" on the people of Northern Ireland.
The EU has told the British Government it must implement the Brexit deal in full — this would mean checks on certain goods moving between Britain and Northern Ireland.
Talks had stalled last week on any movement with he bloc, in a round of talks dubbed the "sausage wars". Brexit minister David Frost said at the time there were "no breakthroughs or breakdowns" with the block on the implementation of the NI Protocol, including treatment of sausages and chilled meats, following "frank and honest" discussions.
Johnson said on Saturday: "I've talked to some of our friends here today, who do seem to misunderstand that the UK is a single country, a single territory. I just need to get that into their heads."
The PM met with French president Emmanuel Macron and German chancellor Angela Merkel on Saturday. He also held talks with European Commission president Ursula von der Leyen and European Council president Charles Michel.
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