Labour says the internal Treasury paper, marked “official sensitive”, warns of new customs checks between Britain and Northern Ireland after Brexit – which the prime minister has claimed would not exist.
Speaking in central London on Friday, Mr Corbyn told journalists: “Today I can reveal further hard evidence that Johnson is deliberately misleading the people”.
He added: “This document is very ominous. There will be other secret reports like this one in every government department that reveal the disastrous impacts of his policies on the safety of the food you eat, on the rights you have at work, on the pollution of the air that we breathe and on the jobs and industries that people work in.
“These reports exist but the government is hiding them from you because in this election the Conservatives want you to vote blind.”
The leaked document, entitled “NI Protocol: Unfettered access to the UK internal market”, appears to be an official analysis of the Northern Ireland protocol, which replaces the Irish border backstop negotiated by Theresa May.
The Labour leader said the confidential report “drives a coach and horses” through Mr Johnson’s promise to the DUP that there will be no border in the Irish Sea under his Brexit plan.
The paper, which has been seen by The Independent, reveals there would be customs declarations and regulatory checks on goods travelling between Northern Ireland and Britain – which Mr Johnson has repeatedly denied.
Checks could be “highly disruptive” to the Northern Ireland economy, where 98 per cent of exporters to Britain are small businesses who will be hit hard by these extra costs, according to the report.
Prices of imported high street goods are likely to rise, which would hit business profits and the retail sector, it added.
Mr Corbyn said: “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.”
The leaked document seems to confirm in the government’s own words what essentially all trade experts say about the Brexit deal with regard to customs, but which Mr Johnson has continued to deny point-blank when asked in public.
At the start of the election campaign, the prime minister was asked in person by Northern Ireland business leaders whether they would be subject to customs declarations.
“You will absolutely not,” he told them. “If anyone asks you to do that tell them to ring up the prime minister and I’ll direct them to put that form in the bin.”
However, Steve Barclay, the Brexit secretary, and Julian Smith, the Northern Ireland secretary, have both previously said that some checks will be necessary on goods travelling across the Irish Sea.
Mr Johnson was challenged over the disparity between the document and his previous claims during a campaign visit in southern England, where a journalist asked him: “Either you don’t know the detail of your policy or you are lying. Which is it?”
He replied: “I know perfectly well that goods going from NI to GB and GB to NI will have no checks. The only checks will be those going via NI into Ireland. Everybody understands that and that’s so we avoid a hard border between Ireland and Northern Ireland.”
Asked what voters should believe, the prime minister said: “They should believe exactly what I say, which is that there will be no checks on goods going from GB to NI or NI to GB, because we are going to come out of the EU whole and entire.
“That’s the objective we secured, which is a vast improvement on the original proposal, which would indeed have carved out Northern Ireland and kept it in the EU for all sorts of purposes.”
Earlier in the campaign, Labour presented separate leaked documents that showed UK negotiators have been repeatedly discussing the NHS with US trade officials, despite claims by Mr Johnson that the NHS is not “on the table” in talks.
A Treasury spokesman said: “We don’t comment on leaked documents.”
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