Nicola Sturgeon predicted a Brexit would "almost certainly" lead to calls for another independence referendum as Boris Johnson became the latest high-profile Tory to back leaving the European Union.

The announcement by the London Mayor added to David Cameron’s woes on a weekend in which a raft of cabinet ministers declared they would campaign against him in June 23’s referendum.

Yesterday the Prime Minister was forced to use a television interview to urge Mr Johnson to back an In vote, warning him against "linking arms with Nigel Farage and George Galloway".

But the MP, tipped as a future Tory leader, is understood to be disappointed by Mr Cameron’s proposals to ensure UK law takes precedence over EU legislation.

Mr Cameron is expected to face questions about those plans and the rest of the reforms he has secured from the EU when he appear before MPs in the Commons today.

Mr Johnson's decision is the second major coup for the Out campaign in as many days, after it secured the support of one of Mr Cameron's close allies, Justice Secretary Michael Gove.

Yesterday. the First Minister predicted there would be an "inescapable" shift in Scottish opinion towards independence as a way to stay in the EU, if there was a Brexit.

Her comments came as Ukip leader and arch eurosceptic Mr Farage challenged her to a TV debate on the EU.

Critics accused the SNP leader of ‘talking up’ the chances of a Out vote – and independence.

Labour also accused the SNP of being 'confused' about why it wanted to stay in one union and leave another.

Asked if a vote to leave, against the will of Scots, would trigger another independence referendum, Ms Sturgeon told the BBC's Andrew Marr Show: "Almost certainly.

"I think that would be the demand of the people of Scotland.

“If a couple of years later we find ourselves, having voted to stay in the European Union, being taken out against our will, I do think there are many people – including people who voted No in 2014 – who would say ‘the only way to guarantee our EU membership is to be independent and that, I think, is inescapable.”

Ms Sturgeon also said she would not appear on the same platform as David Cameron, as she warned the Prime Minister not to campaign in Scotland. “I'm not sure it would help his cause too much, I think he should perhaps think twice about that,” she said. “I remember in the independent referendum we used to be overjoyed every time he made a foray into Scotland to campaign there because we thought it ratcheted votes up for the Yes campaign.”

She also suggested that Mr Cameron’s renegotiation strategy could have increased the chance of a Out vote, saying that many were “disappointed” with the deal he secured in Brussels last Friday.

Despite the predictions of a second independence referendum, she insisted she did not want a Brexit.

“I hope the UK as a whole votes to stay in the EU for a whole variety of different reasons, ” she said.

The EU was one of the biggest issues in 2014’s independence referendum.

The No campaign argued that the SNP could not guarantee that an independent Scotland would not be outside the EU, even for a short period.

Ms Sturgeon said that she personally knew “passionate’ No voters who would change their minds if the UK left the EU.

?Scottish Conservative chief whip John Lamont said: "As always with the SNP, everything comes back to their campaign for independence.”

Labour’s shadow Scottish Secretary Ian Murray, who will give a speech at the Foundation for Progressive Studies in Brussels tonight (MON), said the Tories were ”hopelessly divided” while the SNP were “tying themselves in knots about why we should share sovereignty with our neighbours across the Channel, but not across the Tweed”.

Lib Dem leader Willie Rennie said: "It is no surprise that Nicola Sturgeon favours independence for Scotland but she is too keen to talk up a defeat in the EU referendum to advance her cause."

Meanwhile, Mr Farage appeared to reject a TV debate with Alex Salmond, who last week said he would be “delighted” to take on the Ukip leader.

Mr Farage said that if Ms Sturgeon turned him down then he would ”very seriously” consider Mr Salmond.

Mr Farage also defended sharing a platform with former Respect MPs and London Mayoral candidate George Galloway.

A number of activists walked out of a rally of Brexit campaign Grassroots Out in central London on Friday in protest at Mr Galloway’s surprise appearance on the platform.