Speaking after talks failed to achieve a breakthrough, Maros Sefcovic, the European Commission vice-president, warned that “patience is wearing very, very thin”.
Mr Sefcovic laid bare his frustration after the UK failed to shift ground in the morning talks.
Crucially, he failed to rule out the UK unilaterally delaying the ban on the export of chilled meats from Great Britain – due to come in at the end of this month – the “sausage wars”.
Speaking hours after the talks, Mr Johnson insisted that he was “not worried” that this week’s G7 summit in Cornwall will be overshadowed by the threat of trade war with Brussels, insisting he was “very, very optimistic” that a solution was “easily do-able”.
But he gave no indication of any shift in position in response to Wednesday’s deadlock, saying only that he wanted a solution “that protects the peace process but also guarantees the economic and territorial integrity of the whole United Kingdom”.
The commission vice-president warned that, if further unilateral action is taken, the EU would react “swiftly, firmly and resolutely” to ensure the UK sticks to the protocol it agreed and signed.
“The UK has to abide by its legal obligations and perform these controls,” Mr Sefcovic told a press conference in London.
“Unfortunately, there are still numerous and fundamental gaps in the UK implementation of our agreement. These gaps need to be filled.”
The legal action the EU has already started over the UK shelving earlier promised checks would end up in the European Court of Justice, with the case likely “in early autumn”.
In an interview after the meeting, Lord Frost hit out at Brussels, saying: “What the EU is insisting on is we should operate the protocol in an extremely purist way.
“The reality is that it’s a very balanced document that’s designed to support the peace process and deal with the very sensitive politics in Northern Ireland.”
But Mr Sefcovic insisted the EU was ready to compromise, over the supply of medicines, VAT on second-hand cars, livestock movements and steel tariffs and “many other examples”.
“But we cannot undo the core of the protocol,” he warned, threatening “cross-retaliation” measures that would undo the zero-tariff, zero-quota deal the UK secured at Christmas.
Asked what part of the Brexit deal would be suspended, Mr Sefcovic pointed to “trade parts, services part” and “other measures”, but added: “I wouldn’t specify them right now.
“We really do not have the menu. We didn’t come here to tell them we are going to do this and that precisely, we just told them there is such a possibility – let’s avoid this.”
However, the vice-president said it was “not true” that the EU is discussing an emergency plan to restrict Ireland’s access to the single market to solve the crisis, as reported.
Asked if he hoped for “an intervention from Joe Biden” when the US president arrives at the G7 summit, he pointed out that the administration was “following this very closely”.
Ahead of a face-to-face meeting with Mr Johnson in Cornwall, Mr Biden’s national security adviser said his message would be the need to protect the peace process.
“Whatever way they find to proceed must, at its core, fundamentally protect the gains of the Good Friday Agreement and not imperil that. And that is the message that President Biden will send when he is in Cornwall,” Jake Sullivan said.
The new leader of the Democratic Unionist Party repeated his call for the protocol to be scrapped in the absence of a breakthrough. “The protocol has failed and has only delivered instability and higher prices for Northern Ireland,” said Edwin Poots, who is due to meet Mr Sefcovic next week.
“It is time to remove the internal UK barriers rather than talking about the outworkings of those barriers.
“If Brussels is deaf to the problems, then our own government must act unilaterally to protect Northern Ireland.”
The former head of the government’s Department for Exiting the EU said Mr Johnson’s attempt to play tough with Brussels was “short-sighted and risky”.
Ex-Whitehall mandarin Philip Rycroft told Times Radio the spat over sausages “absolutely” had the potential to damage the UK’s reputation at a time when Brexit means it is dependent on winning trust as a reliable international partner. Other countries will be watching the row “very, very carefully”, he said.
Mr Rycroft said the UK’s position should be taken “with a large dose of salt”, adding: “The UK side knew exactly what rules would apply as it signed up to the Northern Irish protocol. It’s now a question of making it work.”
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