BORIS Johnson has been caught out “lying again," Labour has claimed, after Jeremy Corbyn revealed a leaked confidential UK Government report, which he insisted contradicted the Prime Minister’s claim that there would be no customs border down the Irish Sea under his Brexit deal.

At a press conference in central London, the Labour leader, brandished the document, Northern Ireland Protocol: Unfettered Access To The UK Internal Market, which he said was “cold, hard evidence” that Mr Johnson had misled people about his plan for Britain’s withdrawal from the EU.

He insisted the papers showed there would be customs checks between Britain and Northern Ireland after Brexit.

But the PM, on the campaign trail in Kent, hit back, dismissing the Labour claims as "complete nonsense" and that, under his Brexit deal, the UK would leave the EU as a whole. Tory HQ sought to rubbish the Opposition's revelation as nothing more than a "wild conspiracy theory".

Describing the 15-page Treasury document as “ominous,” Mr Corbyn highlighted how page nine of the report showed the PM’s “damaging deal may have a significant effect on the economies of Scotland and Wales”.

He told journalists: “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.

“This is cold, hard evidence that categorically shows the impact a damaging Brexit deal would have on large parts of our country.”

The Labour leader asserted that the Tory leader’s repeated insistence that there would be no customs border down the Irish Sea was “simply not true”.

Just hours before what many believe could be a make-or-break moment for Mr Corbyn to change the dynamic of the campaign in his favour at the final televised head-to-head leader's debate this evening, Labour high command is focusing its fire on the issue that it believes is the Tory leader's weakest point: truthfulness.

The Labour leader claimed Mr Johnson's Brexit deal would be "disastrous for businesses and jobs all across the UK" but noted how the leaked papers suggested it would be “highly disruptive” to Northern Ireland.

“High street goods are likely to increase in price, which is likely to affect business profitability. If that is how Johnson defines a good deal for Northern Ireland, what has he got in store for rest of us?”

He declared: "We have now caught Johnson red-handed misrepresenting his own Brexit deal."

Mr Johnson has maintained there would be no customs checks between Northern Ireland and Great Britain under his exit terms but Mr Corbyn insisted the document proved this was false.

He referred to page five of the document, which states: "There will be customs declarations and security checks between Northern Ireland and Great Britain."

Mr Corbyn insisted: "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.

"For trade going the other way, from Great Britain to Northern Ireland, there will be all of the above plus potentially damaging tariffs."

Sir Keir Starmer, the party’s Shadow Brexit Secretary, took to social media to condemn the PM. He tweeted: “Boris Johnson has been caught out lying [again] about the damaging impact of his Brexit deal."

But when quizzed about the leaked report on the campaign trail, the Tory leader brushed aside the Labour claims.

He said: "I haven't seen the document you're referring to but that's complete nonsense and what I can tell you is that with the deal that we have we can come out as one whole UK."

Mr Johnson insisted that voters should "believe" him when he said there would be no checks on goods going between Great Britain and Northern Ireland.

"They should believe exactly what I say, which is that there will be no goods going from GB to NI or from NI to GB because we are going to come out of the EU whole and entire, and that was the objective that we secured."

He added: "It's a fantastic deal that will enable us to get Brexit done and move forward, and as it says very, very clearly, Northern Ireland is part of the customs territory of the UK and goods Northern Ireland have unfettered access to Great Britain. And that's what the deal says."

Later, James Cleverly, the Conservative Chairman, hit out at Labour, saying: "Once again, Jeremy Corbyn is brandishing leaked documents that don’t back up his wild conspiracy theories. This is just another desperate attempt to distract from his refusal to take a position on the biggest issue of the day.

“He simply cannot be trusted. He won’t honour the result of the first referendum and now wants to fiddle a second referendum by changing the rules and allowing millions of EU nationals to cast ballots too.

“Labour have been exposed; they are planning to rig our whole democracy in a desperate bid to force us to stay in the EU.

“The Conservatives have negotiated a great new deal with the EU which takes the whole of the UK, including Northern Ireland, out of the EU. The choice is clear: Corbyn’s Labour, with a gridlocked hung parliament and the chaos of two more referendums, or a majority Conservative government that will end the uncertainty get Brexit done and unleash Britain’s potential,” he added.