Boris Johnson must respect the “rule of law” by fully implementing the post-Brexit arrangements for Northern Ireland, EU leaders have said ahead of the G7 summit in Cornwall.

Charles Michel, the president of the European Council, said the behaviour of the prime minister was of increasing concern to EU member states. “It’s paramount to implement what we have decided – this is a question of rule of law,” he said.

The prime minister will hold a trilateral meeting with Michel and the European Commission president, Ursula von der Leyen, in Cornwall.

The framing of the dispute between the UK and the EU as one of respect for the international legal order will chime with the US president, Joe Biden, who arrived in Cornwall on Thursday. Biden is expected to call for both sides to respect the Good Friday agreement.

Under the withdrawal agreement signed by Johnson, Northern Ireland in effect remains in the single market and the EU’s customs code is enforced down the Irish Sea to avoid a hard border on the island of Ireland. But Brussels has complained that these arrangements are not being respected.

The EU has already accused Johnson’s government of breaking international law by unilaterally extending grace periods on a range of controls and border checks on goods entering Northern Ireland from Great Britain.

At a meeting on Wednesday between David Frost, the UK’s Brexit minister, and EU representatives, they failed to find agreement on a range of other disputes and looming crunch points.

The construction of border control posts at ports in Northern Ireland has been suspended, sufficient staff have not been recruited to carry out controls and checks and systems to trace goods have not been built, EU officials complain.

The EU believes the UK’s attitude towards the end of a grace period on a ban on the export from Great Britain to Northern Ireland of chilled meats such as sausages and mince will present a “crossroads” moment.

A further unilateral extension of the grace period would most likely lead to the EU opening a case in the withdrawal agreement’s dispute resolution procedure. That could lead to the application of tariffs on UK goods entering the EU or the suspension of parts of the trade deal.

The EU has not ruled out a jointly agreed extension of the grace period but there is a lack of trust in the UK to help find a permanent solution.

The two sides are also in dispute over the best way to avoid enforcement of the full gamut of sanitary and phytosanitary controls from 1 October on imports from Great Britain on meat, fish, eggs and diary, including time-consuming export health certificates (EHCs), which need to be completed by a vet or other qualified person.

During a joint press conference with Michel ahead of meetings with Johnson in Cornwall, Von der Leyen said the UK could not avoid all of the consequences of Brexit.

She said: “We agreed with the United Kingdom that the protocol was the only solution, ensuring the absence of a hard border for Northern Ireland [with the Republic of Ireland], we’ve been really debating that since years and we found the one and only solution.
“Now, we have a treaty on

that, the withdrawal agreement, it has been signed by both sides – Pacta sunt servanda [agreements must be kept]. It is important that we now implement the protocol. We have shown flexibility, we will show flexibility, but the protocol and the withdrawal agreement has to be implemented, completely.

“The good part about an agreement, a signed treaty, is that both sides signed up to also a dispute settlement mechanism, and to remedial measures that can be taken. So, there are no unilateral actions, but there is an agreed dispute settlement mechanism with different steps.”

However, Von der Leyen stressed she wanted the protocol to work for “everyone” and added that the commission would be flexible in the coming weeks as solutions are sought.