Jeremy Corbyn claims to have obtained a leaked government document which details the "cold hard evidence" Boris Johnson has "misled" voters about his Brexit deal.
In a speech in central London on Friday, the Labour leader brandished a Treasury report which Mr Corbyn said proves there will be border checks between Northern Ireland and the rest of the UK - contrary to claims made by the prime minister throughout the general election campaign.
Mr Johnson said he had not seen the leaked "Northern Ireland Protocol: Unfettered Access to the UK Internal Market" report, and dismissed accusations there would be customs checks as "nonsense".
The Conservatives later dismissed the leaked Treasury document as an "initial assessment" compiled by "junior civil servants" - adding it had never been seen by senior officials, ministers or the prime minister.
Mr Corbyn said Mr Johnson's Brexit deal will be "disastrous for businesses and jobs all across the UK" and claimed the Government's confidential report "confirms this".
"What we have here is a confidential report by Johnson's own government, marked official, sensitive, that exposes the falsehoods that Boris Johnson has been putting forward," Mr Corbyn said.
"This is cold hard evidence that categorically shows the impact a damaging Brexit deal would have on large parts of our country."
Detailing some of the government's alleged own analysis of the impact Mr Johnson's Brexit deal could have on the UK, Mr Corbyn said: "It says this deal will be the equivalent of imposing tariffs of 30 per cent of all purchases made in Northern Ireland.
"That would mean high street goods would increase in price, which is likely to affect business profitability.
Corbyn reveals details of customs checks between Northern Ireland and Great Britain
"If that's how Boris Johnson defines a great deal for Northern Ireland, what impact will it have on the rest of us?
"In fact, it isn't just about Northern Ireland... Johnson's damaging deal may have 'a significant effect on the economies of Scotland and Wales'."
He added: "It is there in black and white. It says there will be customs declarations, absolutely clearly, for trade going from Northern Ireland to Great Britain.
"The Government cannot rule out regulatory checks, rules of origin checks, and animal and public health checks also.
Mr Corbyn said Labour's Brexit general election pledge to give voters a "final say" would see the party negotiate a "credible" deal with the EU which would be put back to the public, without "trashing" the NHS and the economy.
Corbyn says NHS would 'absolutely' be on the table
"Unlike Boris Johnson's fraudulent Brexit, a deal on our terms would respect the priceless peace process in Northern Ireland and protect manufacturing and people's jobs.
"And then we would put that deal to a public vote within six months.
"Those who want to leave the EU without selling out our NHS or trashing our economy will be able to do so."
Johnson dismisses Treasury report as 'nonsense'
Boris Johnson told ITV News Political Correspondent Paul Brand he has not seen the leaked government document, which Mr Corbyn said proved there would be customs checks between Northern Ireland and the rest of the UK.
He said: "I haven't seen the document that you're refering to but that's complete nonsense.
"What I can tell you is that the deal that we have we can come out as one whole UK... together, we can do free trade deals, together, we can take back control of our money, our borders our laws."
When pressed again on the leaked document, Mr Johnson said: "They should believe exactly what I say which is there will be no checks from GB to NI and NI to GB becase we are going to come out of the EU whole and entire and that was the objective.
"It is a vast improvement on the original proposal which would indeed have carved our Northern Ireland and kept it in the EU."
However moments later, Mr Johnson admitted there would be checks but "only for goods that are going via Northern Ireland into Ireland".
It remains unclear how Mr Corbyn obtained the alleged government document, but Conservative Party sources have confirmed the legitimacy of the report to ITV News Economics and Business Editor Joel Hills.
It comes ahead of the final TV head-to-head clash between Mr Johnson and Mr Corbyn days before polling day.