Britain and the European Union should stop trying to score points against each other in the wake of Brexit, the EU's ambassador has said.
Joao Vale de Almeida told reporters that there needed to be "high levels of mutual trust" to make the best of the new relationship between the two parties.
"I think we need to make an effort to change the mindset and give up on trying to score points on disputes of the past and focus ourselves on doing what we can do in making the most out of the agreements that we made," the ambassador said.
His comments come amid a war of words over the Northern Ireland border and threats of legal action by the EU against an alleged breach of the agreement by the UK.
Meanwhile the EU and UK have both stepped up lobbying efforts in Washington DC in a bid to appeal to Joe Biden's new administration over the issue.
It comes as the EU accused the UK of breaking international law by reneging on part of the Brexit agreement, which is designed to prevent a hard border in Northern Ireland.
The UK has said it will unilaterally change the deal to make it more acceptable to UK business – extending a grace period exempting importers from checks without the EU's approval.
The Brexit deal has introduced new bureaucracy for businesses across the UK compared to single market membership - but Northern Ireland has been the hardest hit, with supermarket shortages of basic goods.
The situation is expected to worsen even further when a grace period for supermarket suppliers ends – with the original date set for the end of this month.
The decision to unilaterally extend the grace period has soured the mood in Brussels and EU capitals towards Britain, which is increasingly being described as an unreliable partner.
EU commissioner Maros Sefcovic and Ireland's foreign minister Simon Coveney spoke to an influential group of US congress members on Wednesday about the "tensions" caused by the UK government's behaviour.
The Friends of Ireland caucus and its chairman Richard Neal are widely seen as having the ear of Mr Biden, who has ancestral links to Ireland and has said a US-UK trade deal would be dependent on Brexit not undermining the Good Friday Agreement.
Mr Coveney told Irish public broadcaster RTE: "We talked for about an hour-and-a-half about the protocol, its implementation, the tensions around that, the mistakes that have been made by both sides.
"The need to try to re-engage in discussion, because without finding a way forward through dialogue, which of course has to be the preference for everybody, then Maros Sefcovic outlined that the EU side really has no option but to take legal action, which will begin this week."
The European Commission has said it will take legal action against the UK over the unilateral action in the comings days.
Brexit minister Lord Frost said earlier this week that the UK would "vigorously" defend any legal action brought by Brussels, describing the measures as "operational, technical and temporary".
He also accused the European Union of "ill will" over Brexit.
Meanwhile the UK government has announced that it will be sending its own senior official from the Northern Ireland Office to try to build relations with the Biden administration.
Northern Ireland Secretary Brandon Lewis on Wednesday insisted the move had been "lawful".
"My view is having spoken to businesses, if we hadn't have taken the action that we took last week, we would have had empty shelves in supermarkets in Northern Ireland imminently now," he said.
"I think that would have raised tensions further and it may well have undermined the protocol fatally in a way that is not actually in the best interests of either the EU, the UK or the people of Northern Ireland."
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