Jeremy Corbyn has said a leaked document obtained by Labour shows Boris Johnson is "misrepresenting" his Brexit deal and the "devastating" impact it will have on Northern Ireland.
He called the paper "hard evidence" NI would be "symbolically separated" from the rest of the UK after Brexit, with customs checks on goods.
Mr Johnson has repeatedly said there will be no border in the Irish Sea.
He dismissed Labour's claims as "complete nonsense".
Asked about the document during a campaign visit in Kent, Mr Johnson said he had not seen it but insisted his agreement would offer "unfettered access" to the British market for Northern Ireland businesses.
The Conservatives said the leaked Treasury document was an "immediate assessment, not a detailed analysis".
It had not been written for "decision-making purposes" or been seen by the PM, the chancellor or "any of the senior officials involved in the negotiations", the party added.
But the Democratic Unionist Party (DUP), who oppose the agreement, said it had warned Mr Johnson that it would be bad for Northern Ireland and this was "further evidence".
The row comes as the leaders prepare for the last TV debate of the election on BBC One on Friday at 20:30.
Ahead of the encounter, Labour sought to increase the pressure on the prime minister, who has claimed his agreement with the EU on the terms of the UK's withdrawal will "get Brexit done" by 31 January.
Mr Johnson has said the whole of the UK will leave the EU at the same time and that there will be no checks on any goods moving from Northern Ireland to the rest of the UK.
At a press conference in London, Mr Corbyn said the 15-page Treasury document - titled Northern Ireland Protocol: Unfettered Access to the UK Internal Market - disproved this and showed Mr Johnson's claims about his own deal were "fraudulent".
By John Campbell, the BBC's Northern Ireland business and economics editor
The analysis in this leaked document matches that published in a government risk assessment in October.
The initial study said the PM's deal could mean a reduction in business investment, consumer spending and trade in Northern Ireland. But this time the language is even blunter and confirms the worst fears of unionists.
It concludes that the deal would see "Northern Ireland symbolically separated from the Union."
And it once again suggests the government has not been straight about the extent of new red tape on trade across the Irish Sea. Page one of the document says "at a minimum exit summary declarations will be required when goods are exported from NI to GB".
The prime minister has repeatedly said those declarations would not be required.
"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," he said.
"This is cold, hard evidence that categorically shows the impact a damaging Brexit deal would have on large parts of our country, 15 pages that paint a damning picture of Johnson's deal on the issue of Northern Ireland in particular."
As well as customs and security checks on goods moving from Great Britain to Northern Ireland, he said goods moving the other way would be subject to regulatory and rules of origin checks and potentially tariffs - which would force up prices and be highly "disruptive".
"This drives a coach and horses through Boris Johnson's claim that there will be no border in the Irish Sea."
Before he succeeded Theresa May as prime minister, Mr Johnson told the DUP's annual conference in 2018 that no UK government could agree to any border in the Irish Sea. Mr Corbyn said this showed Mr Johnson's word could not be trusted.
But the prime minister said his agreement was superior to his predecessor's as it would give the Stormont Assembly the power to decide whether to remain aligned with the EU after four years.
Mr Johnson said the only checks would be on British exports to the Republic of Ireland going via Northern Ireland.
BBC Reality Check correspondent Chris Morris said this was not correct, as the agreement actually envisaged checks at Northern Irish ports on British goods not formally bound for the Republic, with items "at risk" of being transported on there being liable for duties.
This Treasury document sets out things that trade experts have been saying pretty clearly, but that the government has refused to accept.
Boris Johnson's EU Withdrawal Agreement says customs declarations and documentary and physical checks on trade between Great Britain and Northern Ireland will be "highly disruptive to the Northern Ireland economy".
The document notes that 98% of businesses that export to Great Britain are small and medium businesses that are "likely to struggle" to bear the cost.
None of this is a huge surprise to anyone committed enough to have read the Protocol on Ireland and Northern Ireland carefully, but it is not what the prime minister has been saying about his own deal.
Another striking line from the leaked document says the withdrawal agreement "has the potential to separate Northern Ireland in practice from whole swathes of the UK's internal market".
That is why, in a nutshell, Mr Johnson lost the support of the Democratic Unionist Party in Northern Ireland, and it was his lack of a working majority in Parliament that led in turn to this.
Under the PM's agreement, Northern Ireland will continue to follow many EU rules on food and manufactured goods, while the rest of the UK will not. Northern Ireland will also continue to follow EU customs rules but will remain part of the UK's customs territory.
A government risk assessment published in October said it would lead to new administration and checks on goods from west to east.
But Mr Johnson has insisted Northern Irish businesses will not be hit with additional paperwork or fees, telling a BBC phone-in during the campaign that "we will make sure that businesses face no extra costs and no checks for stuff being exported from NI to GB".
The DUP said it would use whatever influence it had in the next Parliament to push for changes to the agreement.
"Despite the prime minister's protestations, the facts are in black and white," said spokesman Sir Jeffrey Donaldson. "We have demonstrated over the last three years that we will stand up and speak up for Northern Ireland to ensure our economy is not decimated by a bad deal."
Mr Corbyn said Labour's promise of another referendum within six months of his winning power would "end the division" over Brexit as well as protecting jobs and the peace in Northern Ireland.
Under Labour's plans, voters would get to choose between a "credible" renegotiated Leave deal, including a customs union with the EU and a close single market relationship, and staying in the EU under current terms.
However, Mr Corbyn again declined to say which way he would vote - saying he would not "take sides" so he could faithfully carry out the result. In contrast, his chief Brexit spokesman Sir Keir Starmer said he would campaign against the new deal Labour negotiated and back remain.
Conservative Party chairman James Cleverly said: "Once again, Jeremy Corbyn is brandishing leaked documents that don't back up his wild conspiracy theories."
But Liberal Democrat Brexit spokesman Tom Brake said the document showed Mr Johnson's Brexit deal "would be in fact a knockout blow to the economy of Northern Ireland".
SNP foreign affairs spokesman Stephen Gethins said it made it "clear" that "Scotland will take a disproportionate hit from Boris Johnson's disastrous Tory Brexit deal".
Speaking in Dublin, EU Trade Commissioner Phil Hogan said that under Mr Johnson's Brexit deal Northern Ireland would "remain in the UK customs territory and, at the same time, benefit from access to the single market without tariffs, quotas, checks or controls".
"EU state aid and VAT rules will continue to apply in Northern Ireland, under the control of the European Court of Justice," he added.