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.