BORIS Johnson has accused backers of a second EU referendum of “threatening the Union” by giving aid and succour to the SNP.

The former Foreign Secretary said former Tory Prime Minister Sir John Major and Labour PM Tony Blair “should know better” than play into Nationalist hands.

Mr Johnson, who fancies himself as a Brexiter occupant of Number 10, was speaking at a Tory fundraising dinner in Aberdeen, in the most Eurosceptic corner of Scotland.

He said a second independence referendum would be “nothing less than a flagrant breach of promise to the people of this entire country”and the No vote of 2014 should be “respected for a generation”.

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He said: “Frankly, I say to all those now campaigning for a second EU referendum in the UK - I mean Tony Blair, Alastair Campbell, and I am afraid not only John Major but even some cabinet ministers have canvassed the idea - they should know better.

“By militating for EUref2 or a People’s Vote, they are giving aid and succour to those who would stage another independence vote in Scotland. By undermining the verdict of the EU referendum, they are undermining the decision of 2014, they are doing the work of the SNP, and they are threatening the Union.

“Let us be in no doubt about the damage the Nationalists would do.

“They would not only destroy the oldest and most successful political union in the world, they would destroy the very concept of Britain.”

Mr Johnson’s appearance was not supported by the Scottish Tory leadership, who fear he would repel Scots voters if he replaced Theresa May in Downing Street.

However Ruth Davidson’s party did have reason to celebrate after holding off a Labour challenge to win the Haddington and Lammermuir byelection on East Lothian Council.

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Former journalist Craig Hoy held the seat for the Tories with 35 per cent of the vote, ahead of the SNP, as Labour slumped 12 points to 22%, despite the area having one of Scotland’s biggest Labour branches.

A Labour source said: “This is a devastating result that is no reflection at all on an incredible local campaign. The blame lies squarely with the leadership. A dramatic change of policy in full support of a People’s Vote is now urgent or we’ll be annihilated in the Euro elections.”

Ms Davidson said voters were “deserting the Labour Party in Scotland”.

She said: “This is a fantastic result for the Scottish Conservatives and I know Craig Hoy will be a superb, hard-working local councillor for Haddington and surrounding villages.

“With the Scottish Conservatives topping the poll, it provides further evidence that voters are deserting the Labour Party in their droves.

“Even in areas where they were once strong, people now view Labour as too weak to take on the SNP.

“It is only the Scottish Conservatives which offer a positive message to take Scotland forward and bring the country back together.”

The by-election was sparked by the resignation of Conservative councillor Brian Small.

Mr Hoy said his victory showed “that the clear dividing line in Scottish politics is between us and the SNP”.

He said: “By focusing on local issues and warning of the dangers of a second independence referendum we’ve secured a clear victory over Labour and the SNP in Haddington and the Lammermuirs.

“It’s increasingly clear that only the Scottish Conservatives stand up for local people here in East Lothian and across Scotland as whole.”

More than 44% of voters took part in the by-election, which was crucial in deciding whether the Scottish Tories would retain their position as the official opposition.

East Lothian Council is run by a minority Labour administration.