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Scottish Conservative leader Ruth Davidson will declare independence from her UK party if Boris Johnson becomes its leader, The Courier can reveal.

David Cameron has already said he will not fight a third election as Tory leader but some at Westminster are predicting a coup in the aftermath of Thursday’s EU referendum, particularly if there is a Leave vote.

The prospect of Mr Johnson replacing him as Prime Minister is causing such concern in the Scottish top team that going solo, in a move which would mirror Murdo Fraser’s leadership campaign tactic, is seen as a viable option.

The Mid Scotland and Fife MSP was defeated in the 2011 contest to take charge of the party after proposing the Scottish Conservatives must be replaced with a new right wing group.

Ms Davidson, who saw off Mr Fraser’s challenge five years ago and guided the Tories to second place in May’s Holyrood election, is now considering such radical action ahead of her televised showdown on the EU referendum with the former London Mayor on Tuesday evening.

A source close to Ms Davidson said: “If Boris becomes leader we’ll do a Murdo. We’ll have to break off.”

Supporters of Mr Fraser’s plan last night argued that forming a new party would be the “next logical step” for her, whether Mr Johnson entering Downing Street or another factor was the catalyst.

Party insiders said she had distanced the Scottish party from the UK Tories in several key policy areas and they campaigned in the recent Holyrood election under the banner “Ruth Davidson for a strong opposition” rather than the Conservative name.

There is no love lost between the Ms Davidson and Mr Johnson, which is likely to make for a lively event at Wembley for both those in the stadium and the millions of TV viewers.

© PA

She savaged her opponent in an article for a tabloid newspaper, arguing a Leave Vote would be “a conscious decision to make Britain poorer” which “would hurt the poorest the most”.

She added: “Boris Johnson and Nigel Farage would be OK; the wealthy are always able to fall back on their pension pots and savings. It would be ordinary workers who would suffer: the easyJet air hostess who could lose her job because, after Brexit, the airline would be priced out from flying within Europe; the dad on the factory floor at one of our many car-makers whose job disappears because Europe has slapped a new tariff on British-made motors; the single mum on a zero-hours contract whose job is extinguished to cut costs.”

She also compared Mr Johnson and Ukip leader Mr Farage’s Leave strategy to “brazen chauvinistic style of (Alex) Salmond.”

Mr Johnson will lead a three-person team arguing for Brexit alongside Labour MP Gisela Stuart and Tory energy minister Andrea Leadsom.

The event, on BBC One at 8pm, will pit them against Ms Davidson, Sadiq Khan, the Labour London Mayor, and Frances O’Grady, general secretary of the Trades Union Congress.

The Scottish Conservatives have denied the story as Ms Davidson tried to back pedal.