Northern Irish MPs have vowed to fight any bid to make the Irish Sea the post-Brexit border with the United Kingdom amid reports that Dublin believes proposals to create a frictionless land border are unworkable.

Leo Varadkar, the Irish prime minister, reportedly wants the Irish Sea to be made the border with customs checks moved to ports and airports.

But Sir Jeffrey Donaldson, a senior Democratic Unionist MP, said there was “no way” his party would sign off on the plan amid concerns that it would create a barrier between Northern Ireland and the rest of the UK.

The DUP’s opposition to the sea border proposal could spell a headache for Theresa May as she seeks to hammer out a Brexit deal with her minority Government reliant on the party’s MPs to pass laws in the House of Commons.

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The Government has suggested using surveillance cameras to allow free trade and movement to continue between Northern Ireland and the Republic as part of a bid to ensure there is not a return to the hard border of the past.

But The Times reports that Dublin does not believe this to be sufficient and wants checks to be moved to ports and airports, effectively making the Irish Sea the dividing line between the two land masses.

Simon Coveney, the Irish foreign minister, reportedly told European counterparts:"What we do not want to pretend is that we can solve the problems of the border on the island of Ireland through technical solutions like cameras and pre-registration and so on. That is not going to work.”

But Sir Jeffrey told BBC Radio 4’s Today programme: “We would be strongly opposed to the idea that you would create a border in the Irish Sea between the island of Ireland and Great Britain.

“Pragmatically that is just not going to happen. If you look at Northern Ireland, for example, out of the port of Belfast 73% of the goods that come in and out of the port of Belfast which is by far our busiest port go to Great Britain.

“Why on earth would we want to create a customs arrangement between this part of the United Kingdom and the rest of the United Kingdom.

Prime Minister Theresa May with DUP leader Arlene Foster (left), as DUP MP Sir Jeffrey Donaldson (second right) and Parliamentary Secretary to the Treasury, and Chief Whip, Gavin Williamson, signing paperwork inside 10 Downing Street, London, after the DUP agreed a deal to support the minority Conservative government credit: Daniel Leal-Olivas/PA

“That just isn’t going to be the outcome and I think the Taoiseach needs to get a reality check on this.”

When asked if he could rule out his party’s support for such a proposal, he said: “Yes, I certainly can. There is no way that the DUP would go for an option that creates a border between one part of the United Kingdom and the other.

“Dublin really needs to understand that that proposition is absurd. It is unconstitutional.

He added: “Why would we want to create a customs situation with the very market that is our biggest market? It’s just madness.”