This article is more than 9 months old

This article is more than 9 months old

Labour has accused Boris Johnson of misrepresenting the impact of his Brexit deal for Northern Ireland, as Jeremy Corbyn revealed a leaked Treasury document that suggested prices will rise and businesses will struggle to bear the costs of border checks.

The Labour leader said the document revealed the “cold, hard facts” about the impact of Boris Johnson’s Brexit deal on Northern Ireland.

The document, entitled Northern Ireland Protocol: Unfettered Access to the UK Internal Market, warns that “the withdrawal agreement has the potential to separate Northern Ireland in practice from whole swathes of the UK’s internal market”.

Corbyn said: “This drives a coach and horses through Boris Johnson’s claim that there will be no border in the Irish Sea.”

The document suggests that for trade going from Northern Ireland to Great Britain, the government cannot rule out several different checks, including on regulations and animal health.

“At minimum, exit summary declarations will be required when goods are exported from Northern Ireland to Great Britain in order to meet EU obligations,” it says.

And for trade going the other way – from Great Britain to Northern Ireland – there could potentially be tariffs, the document suggests.

A section headed “economic impact on Northern Ireland” suggests high street goods are “likely to increase in price”, and many exporters could struggle with the costs of border checks, which will be “highly disruptive”.

Get an evening roundup of events on the election campaign trail from Guardian political correspondent Andrew Sparrow

The prime minister has repeatedly insisted the Brexit deal he struck in November will not require border checks. He even told one business in Northern Ireland that if they were asked to fill in a form, they should ring him, and “I will direct them to throw that form in the bin”.

The shadow Brexit secretary, Keir Starmer, appearing alongside Corbyn at the press conference in London, said: “The issue of whether there are going to be checks is absolutely blown out of the water by this document, as is the suggestion that it won’t be bad for business.

“This exposes the untruth of Johnson and the evidence is coming from his own Treasury.”

The document appeared to have been drawn up to set out the economic and political implications of the government’s promise to maintain “unfettered access” to the British market for business in Northern Ireland.

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That promise is made in the Northern Ireland protocol that replaced the backstop in Johnson’s Brexit deal.

At the event in central London, Corbyn also appeared to acknowledge that with less than a week to go until polling day, he was a divisive figure.

Asked whether he believed another leader could be more successful among working-class voters, he said: “I think Marmite’s really good for you; some people like it, some people don’t.”

Starmer, who has made few frontline media appearances during the campaign, confirmed that he would support remain in a second Brexit referendum, which Corbyn has promised to hold within six months. “I voted for remain last time and I’d do it again,” he said.

At a Conservative campaign event near Maidstone in Kent, the prime minister was asked about the documents. He said he hadn’t read them but that Labour’s claims were “nonsense”.

“What I can tell you is that with the deal that we have, we can come out as one whole UK – England, Scotland, Wales, Northern Ireland, together. We can do free-trade deals together, we can take back control of our borders and our own immigration system.

“If you look at Jeremy Corbyn’s proposed deal, under no circumstances are he or his colleagues going to take back control of immigration.”

He added: “They should believe exactly what I say, which is that there will be no checks on goods going between GB to NI, or NI to GB, because we are going to come out of the EU whole and entire.”

Conservative sources also sought to insist that the documents were not complete. However, Starmer insisted: “He has either not read the advice from the Treasury or isn’t telling the truth. Probably both.”

A senior government source said the document produced by Labour was an “initial appraisal” compiled by junior civil servants and had not been signed off at a senior level. The Conservative party said the document had been produced immediately after Johnson’s deal was struck and was not written or used for decision-making purposes.

They pointed to a number of question marks in the document, which they said showed it was an incomplete analysis. They added that the deal was fully compliant with the Belfast/Good Friday agreement.