Boris Johnson has been accused of planning to renege on the terms of his own Brexit deal, after telling Northern Irish businesses that he will not enforce checks arising from the proposed customs border in the Irish Sea.

Mr Johnson also sparked controversy by telling Tory members that Northern Ireland had a “great” Brexit deal as it retains access to the single market and freedom of movement – something that the rest of the UK will lose under his EU withdrawal plan.

The prime minister was accused of failing to understand the details of his Brexit deal, after being caught on camera telling Northern Irish businessmen that their exports to mainland Britain would not face any extra bureaucratic checks as a result of the proposed customs border in the Irish Sea.

But he later said that, while arrangements for the checks – known as exit summary declarations – might be put in place, he would simply not require them to be enforced.

Speaking to Conservative members during a general election campaign visit to Northern Ireland, Mr Johnson insisted there will be “no forms, no checks, no barriers” for goods travelling from the province to the mainland under his deal, assuring them: “You will have unfettered access.”

Asked by one businessman if he could tell staff to refuse to fill in the forms, which would increase the cost of exporting, Mr Johnson said: “You can. If somebody asks you to do that, tell them to ring up the prime minister and I will direct them to throw that form in the bin.”

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His remarks directly contradicted both Northern Ireland secretary Julian Smith and Brexit secretary Steve Barclay, who said last month that businesses in the province would need to fill out export declaration forms when sending goods to the mainland.

Shadow Brexit secretary Keir Starmer accused the prime minister of “making it up as he goes along”.

“This is a prime minister who either doesn’t know the details of a deal that he has negotiated or isn’t being straight about it, or probably both,” said Sir Keir.

“We’ve seen this so many times from Boris Johnson, this sort of casualness with the details – pretending there won’t be checks when it’s written into the treaty, pretending that as prime minister he could somehow waive the checks, when it’s a legal obligation.”

Challenged later in a TV interview over whether he was saying that while new checks would be put in place “you are simply not going to enforce them at all”, Mr Johnson replied simply: “That’s right.”

The Northern Ireland border was one of the key stumbling blocks to securing a Brexit deal for both Theresa May and Mr Johnson, and the prime minister’s apparent readiness to ignore one aspect of the proposed new arrangements risked reopening disputes over the agreement.

Labour peer Stewart Wood said Mr Johnson was ripping up the terms of his own Brexit deal.

“So now we know how the PM will get round the problem of the EU Customs Code requiring NI businesses to complete exit declaration forms when they trade with the rest of the UK,” said Lord Wood. ”He will ignore the requirement and renege on the deal he negotiated. Bold.”

The former adviser to Gordon Brown added: “It seems he plans not to enforce the EU Customs Code requirement – accepted under the terms of his own Brexit deal – that NI firms will have to complete exit declaration forms when they send goods to the rest of the UK.”

As the footage emerged, critics demanded to know why the prime minister was dragging Britain out of the EU if he believed the terms of its membership were so beneficial.

In a clip posted by Manufacturing Northern Ireland, the prime minister said: “Northern Ireland has got a great deal. You keep free movement. You keep access to the single market but you also have, as it says in the deal, unfettered access to GB.

“We can also come out and do free trade deals. The only reason they gave us that deal, by the way, was because at the back of their minds, they were still worried we would come out without a deal.”

Tom Brake, the Liberal Democrat Brexit spokesman, said: “The single market and freedom of movement are a great deal – even Boris Johnson recognises this – so why isn’t he keeping them for the whole of the UK as part of the many benefits of EU membership?

“It is clear that the best deal for the UK is the one we have now: in the EU.”

Labour MP David Lammy said: “Boris Johnson describes keeping free movement and access to the single market as a ‘great deal’ for Northern Ireland.