Joe Biden will not believe Boris Johnson’s “nonsense” claim that the government underestimated how difficult trade would be for Northern Ireland under the terms of its own Brexit deal, Labour has said.

Shadow foreign secretary Lisa Nandy told HuffPost UK’s Commons People podcast that the government’s “excuse” for the region’s post-Brexit settlement was “pure and utter drivel”.

Tensions between the UK and EU over the way the Brexit deal affects Northern Ireland are threatening to overshadow Johnson’s first face-to-face talks with Biden on Thursday and the subsequent G7 summit in Cornwall over the weekend.

The US president has already taken the extraordinary step of ordering officials to deliver a formal diplomatic rebuke to the UK for imperilling the Northern Ireland peace process over Brexit, according to the Times.

Johnson and his Brexit minister Lord Frost want the EU to be less “purist” about applying checks on goods entering Northern Ireland from Great Britain, but Brussels is threatening a trade war with the UK unless it properly implements the deal it signed up to.

Biden is expected to use Thursday’s meeting to urge the PM to “stand behind” the Northern Ireland protocol that he negotiated, but which has triggered the dispute with the EU.

Nandy said Biden is unlikely to believe Frost’s excuse that the UK “underestimated” the impact of the protocol on Northern Ireland.

She told Commons People: “It’s just utter abject nonsense.

“It’s just pure and utter drivel, lies, nonsense – they knew exactly what they were doing and they made big, big promises around it.

“But as always with Brexit they were much more interested in defending their own interests in the Tory Party than they were in thinking about the interests of the country.

“And if I sound angry, that’s because I am”. 

Asked whether Biden would believe the government’s claims, she added: “I don’t think anybody at the moment thinks that there is an excuse for where we’ve ended up.

“But all minds now including ours are concentrated on how we move this forward.”

Nandy said foreign ministers around the world believe Johnson can go “missing in action” at important moments and that Biden would be asking the PM to now “step up”, while acknowledging that “goodwill and creativity on both sides” is required to solve the impasse.

“I speak to counterparts around the world on quite a regular basis,” she said.

“The number of times that people have said to me the prime minister just seems to be missing in action at the moment that it matters.

“We saw violence flare up agian in Belfast recently, and there was just a sense that the prime minister simply wasn’t there.”

She said that Labour had urged the PM to convene an intergovernmental conference, which had support from all sides in Northern Ireland and the Irish government “and yet the government just wasn’t willing to step up and show that level of leadership”.

“And I think that Joe Biden will be asking for that from the prime minister – this is a mess that he created, he said it wasn’t going to be like this and I think everybody, even privately, the government now understands that this is a situation that they’ve got to sort out,” Nandy said.

“We can’t afford to be in a situation where we are constantly at war with our closest neighbours [the EU].

“Because you’ve seen over the last few months since president Biden was elected that the idea that you can have a very antagonistic relationship with the EU but then you can have a very strong relationship with the United States is for the birds.”

The Times reported that the president – who is intensely proud of his Irish roots – took the extraordinary step of ordering the United States’ most senior diplomat in London, Yael Lempert, to deliver a demarche – a formal protest – in a meeting with Frost on June 3.

The newspaper reported that government minutes of the meeting said: “Lempert implied that the UK had been inflaming the rhetoric, by asking if he would keep it ‘cool’.”

The US charge d’affaires indicated that if Johnson accepted demands to follow EU rules on agricultural standards, Biden would ensure that it would not “negatively affect the chances of reaching a US/UK free trade deal”.

Talks between Frost and the European Commission’s Maros Sefcovic on Wednesday failed to make a breakthrough on the protocol.

The EU has threatened to launch a trade war against Britain if it fails to implement checks on goods entering Northern Ireland under the terms of the Brexit “divorce” settlement which Johnson signed.

But Frost has refused to rule out unilaterally delaying the imposition of checks on British-made sausages and other chilled meats due to come into force at the end of the month.

At a press conference in Brussels on Thursday ahead of the summit, European Commission president Ursula von der Leyen again insisted the protocol was the “only solution” to prevent a hard border between Northern Ireland and the Republic of Ireland and must be implemented in full.