Boris Johnson has been accused of misleading parliament after he claimed there would be "no checks" between Northern Ireland and Britain after Brexit.

The prime minister directly contradicted the Brexit Secretary, Steve Barclay, who admitted on Tuesday that Northern Ireland businesses will be forced to fill out export declaration forms when sending goods to Britain under the terms of the new Brexit deal.

His claim was also at odds with Julian Smith, the Northern Ireland secretary, moments earlier, who told a Commons committee there would be checks but they would be kept at a "minimal" level.

Unionists have reacted with fury to details of extra costs and customs checks for businesses in Northern Ireland under Mr Johnson's new Brexit plan, with DUP MPs describing the plan as "absolutely despicable".

The row centres on trade between Northern Ireland and Great Britain, with HRMC estimating that businesses could be charged between £15 and £56 for export declaration forms.

During a prime minister's questions exchange with Labour leader Jeremy Corbyn, Mr Johnson said: "The UK is preserved whole and entire by these arrangements and indeed the whole of the UK will be allowed to come out of the EU customs union so we can do free trade deals together.

"There will be no checks between Northern Ireland and Great Britain and there will be no tariffs between NI and GB."

Labour MP Stephen Doughty said: "Surely Johnson just misled the house there...?

"He claimed there would be “no checks” between NI and GB. Barclay admitted the complete opposite."

The prime minister's spokesman described the plans as a "minimal administrative process" when pressed by journalists on Mr Johnson's comments.

He said: "The fact is that Northern Ireland is and will remain in the UK customs territory.

"The deal explicitly allows the UK to ensure unfettered market access to goods moving from Northern Ireland to GB. We don't intend to carry out checks on the movement of goods.

"There will be a minimal administrative process provided electronically."

The row after Mr Barclay told peers on Tuesday: "Just to be clear, exit summary declarations will be required in terms of NI to GB".

On Wednesday, Mr Smith tried to reassure unionists that Northern Ireland will be "out of the European Union, out of free movement rules, out of the EU budget, out of fisheries policy, out of the common agricultural policy and free to do trade deals".

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However he admitted there would be checks, saying: "We're talking about a very limited set of goods areas and on that I am going to be working with Northern Ireland businesses and political parties to make sure that the government is minimising any challenge between NI and GB."

Pressed on what those checks will be, Mr Smith responded: "There will be some information required but it's a minimal amount."

DUP MP Ian Paisley told him: "I have to go out and tell businesses in my constituency that I don't know what you have to do to carry out £18 billion worth of trade with our country, our country, not a foreign country ... you want me to vote for this? You can't be serious."

Mr Smith replied: "We have got our own commitments we are going to put into the withdrawal agreement bill to deliver unfettered access from GB to NI. We'll be looking at communication technology and we'll be making sure we support Northern Ireland businesses.

"We will be fully committed to making sure that business transaction, the movement of those goods will be as easy and straightforward as we can do. It's obviously in the government's interests that's the case."

But Mr Paisley hit back at the minister, saying he would feel "pretty aggrieved" if he had to fill in a form to move goods from his constituency in North Yorkshire to London.