Boris Johnson’s proposed Brexit deal will lead to additional “strain” between Scotland and the rest of the UK, former prime minister Tony Blair has warned.

The ex-Labour leader said the “separate treatment” of Scotland and Northern Ireland would “not go unremarked”.

But he added that the “Brexit fanatics” within the Conservative Party “appear indifferent” about the impact the policy would have on the Union.

To counter the case for independence, which has enjoyed a rise in support in some recent polls, Mr Blair said there needed to be a “strategy which celebrates the Union of the UK”.

He branded Brexit an “existential threat to the United Kingdom”, saying: “It imposes a strain on the tensile material holding us together.

“It gives those who would break it an additional point of weight. I say that as a convinced Unionist who will remain so, even if Brexit happens.”

Mr Blair, speaking at an event organised by the think tank Reform Scotland, added: “The proposed Johnson deal adds to that strain.

“It envisages Northern Ireland staying within the European trading structure of the single market, even as Britain, including Scotland, leaves it, despite the clear will of the Scottish people.

“The reasons for the Union between England and Scotland remain powerful. But the separate treatment of Northern Ireland, when Scotland too has substantial interests in staying within Europe’s trade zone, will not go unremarked.”

Mr Blair, speaking in Edinburgh, said the UK was in a “perilous situation”, with the country now due to leave the EU in just over three weeks’ time.

Prime Minister Boris Johnson (Victoria Jones/PA)

He said the “march to the extremes” in both the Conservative and Labour parties, under Mr Johnson and Jeremy Corbyn respectively, had been a “disaster for the country”.

While the UK Government is “essentially mimicking the Brexit Party”, Mr Blair claimed Labour had failed to provide a “coherent and unifying opposition”.

He called for politics to return to the centre ground, describing this as a “place of reason” and “maximum agreement”, adding it was “the place where radical change is pursued, but of the practical and sensible kind, the kind which works”.