The European Union has formally launched legal action against the UK for unilaterally extending the post-Brexit grace periods to trade in Northern Ireland.
In an escalation of tensions, the European Commission was preparing on Monday to take the move over the Government’s alleged breach of the Northern Ireland Protocol.
The EU has suggested the UK may be in breach of international law for extending the light-touch regulatory periods, that were due end this month, until October.
The grace periods cover areas such as supermarket supplies and parcel deliveries to Northern Ireland from Great Britain and mean checks are not yet fully applied.
Cabinet Office minister Lord Frost and EU Commission vice president Maros Sefcovic were understood to have spoken on Monday.
EU vice president Maros Sefcovic accused the UK of violating international law and undermining trust by unilaterally extending the grace periods on Northern Ireland.
In a statement, he said: "The Protocol on Ireland and Northern Ireland is the only way to protect the Good Friday (Belfast) Agreement and to preserve peace and stability, while avoiding a hard border on the island of Ireland and maintaining the integrity of the EU single market.
"The EU and the UK agreed the Protocol together. We are also bound to implement it together.
"Unilateral decisions and international law violations by the UK defeat its very purpose and undermine trust between us. The UK must properly implement it if we are to achieve our objectives.
"That is why we are launching legal action today. I do hope that through the collaborative, pragmatic and constructive spirit that has prevailed in our work so far on implementing the Withdrawal Agreement, we can solve these issues in the Joint Committee without recourse to further legal means."
Downing Street was yet to receive a letter notifying it of the infringement proceedings on Monday morning, but one was expected to be dispatched later in the day.
The Prime Minister has said he is looking forward to discussions with the UK's "EU friends" over what he described as sensible temporary measures linked to the Northern Ireland Protocol.
Asked during an official visit to Coventry why problems with the EU had arisen so shortly after a deal was reached, Mr Johnson said: "We haven't seen the EU's letter yet.
"But I think what I would say to our friends in Brussels is very simple - the protocol is there to uphold and to guarantee, to buttress the Good Friday Agreement.
"It (the protocol) should guarantee not just trade and movement north-south but east-west as well.
"That's all we're trying to sort out with some temporary and technical measures which we think are very sensible.
"But obviously we'll look forward to our discussions with our EU friends and see where we get to."
The Northern Ireland Protocol of the Withdrawal Agreement was designed by the UK and EU to avoid a hardening of the border on the island of Ireland when the post-Brexit transition period ended on December 31.
Northern Ireland remained part of the EU’s single market for goods, meaning products arriving from Great Britain face EU import regulations.
Latest political analysis from Tracey Magee: