The prime minister had claimed that there would be no checks or controls on goods crossing the Irish Sea under the treaty he negotiated with Brussels.
Mr Johnson was accused of lying to voters after he repeatedly insisted that there would be no checks, despite the deal’s actual content.
But on Monday, Ireland’s foreign minister Simon Coveney intervened, telling public broadcaster RTE that it was “clear” that the new deal includes checks.
“It was very clear when the deal was done,” Mr Coveney said. “The EU has made it clear they want to minimise the impact on goods coming from Great Britain to Northern Ireland, but at the same time goods coming from Great Britain to Northern Ireland will need to have some checks to ensure that the EU knows what is potentially coming into their market through Northern Ireland.
He continued: “You know, we spent many, many hours of discussion in terms of trying to get that right.”
“Goods going the other way from Northern Ireland into Great Britain will have far less requirement for checks at all, in fact it will probably be limited to an export declaration, because of course that is a matter internally for the UK.”
“So, there was always a distinction between goods coming from Great Britain into Northern Ireland versus goods going from Northern Ireland into Great Britain and we spent many hours discussing and negotiating that, and I think explaining it too.”
Labour last week unveiled a new leaked documents which Jeremy Corbyn said showed Mr Johnson was “deliberately misleading the people”.
DUP leader Arlene Foster meanwhile hammered Mr Johnson, accusing him of having broken his word to unionists to prevent checks within the UK.
But Mr Johnson stuck to him false claim, suggesting that the Treasury document produced by Labour was “wrong”.
He told a BBC phone-in during the election campaign: “We will make sure that businesses face no extra costs and no checks for stuff being exported from Northern Ireland to Great Britain.”
Under the agreement negotiated by Mr Johnson with Brussels, Northern Ireland would continue to follow most EU rules on food and manufactured goods, with the rest of the UK not doing so.
Northern Ireland would also continue follow EU customs regulations but would technically remain part of the UK’s customs territory. The government’s own public risk assessment says there would be extra checks and administration applied to goods coming from Northern Ireland to the UK.