BORIS Johnson is the “most Unionist Prime Minister” of modern times, Jacob Rees-Mogg has insisted, claiming Brexit would strengthen the Union of the United Kingdom.

His comments came as Unionists in Northern Ireland warned that the new Brexit deal would undermine the integrity of the UK and boost Nationalism in Scotland.

The Commons Leader sought to knock back concerns that not only a regulatory border would be established in the Irish Sea but a customs one too.

Sammy Wilson, the DUP’s Brexit spokesman, decried how the new deal would “siphon off” Northern Ireland from the rest of the UK, meaning the province would face extra costs and red tape, and that the voting safeguards in the Belfast Agreement had been “overturned”.

Appealing for Tory MPs to “protect the Union,” the East Antrim MP warned: “This does have an impact on the unity of the UK and will spark further Nationalist sentiment in Scotland.”

Steve Aiken, the Ulster Unionists’ chief whip at Stormont and the party’s finance spokesman, referred to Mr Johnson as the “so-called leader of the Conservative and Unionist Party,” saying he was “dividing up the UK”.

He declared: “I don’t see how anybody who believes in the Union of the UK can vote for Boris Johnson’s deal.”

Mr Aiken told the BBC Radio 4’s World at One that Northern Ireland would not only have to suffer a regulatory border down the Irish Sea, which Theresa May had agreed to, but also a customs border down the Irish Sea. “That doesn’t work for anybody,” he added.

But Mr Rees-Mogg, noted how Lord Trimble, the Tory peer who is a former leader of the UUP, was supporting the Johnson deal. “To say Lord Trimble is not a Unionist is an eccentric view,” he declared.

The Commons Leader explained: “The deal is good for the whole of the United Kingdom and will be good for Northern Ireland as well. It will only have extra arrangements with the EU by its own democratic consent; so there is a democratic lock on it. And, ultimately, leaving the EU has always been about democracy.”

He explained that goods from the UK going onto the Republic would be subject to customs checks but those just going to Northern Ireland would not. The UK Government would be the “determining voice” as to what was checked or not, he stressed.

Mr Rees-Mogg said the deal reflected Northern Ireland’s “special position” and pointed to the devolved settlement, stressing how this had different effects in different parts of the UK such as the Scottish Government setting its own tax regime.

“So, having within the devolution settlement some differences is perfectly reasonable and does not undermine the principle of the UK nor does it undermine the position of N Ireland.”

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The Somerset MP declared: “Leaving the EU will strengthen the UK because of the advantages of the UK economic model will become so abundantly clear over the EU’s economic model and that will encourage Northern Ireland to be more Unionist.”

He sought to reassure his Unionist colleagues in the province by insisting: “Boris Johnson is the most Unionist Prime Minister that we have had in recent times and he is personally completely committed to the Union and that is why he made himself Minister for the Union.”