THE UK runs the risk of rupture if Nicola Sturgeon and Alex Salmond wear the "tartan trousers" in a Labour-SNP alliance that runs the next Government, Boris Johnson has warned.

Making a stronger showing on the campaign trail, the London Mayor said the "brute fact" of the election was that Labour had collapsed in Scotland and the situation was irreversible.

The Tory leading light - who earlier this week came in for criticism for comparing the Nationalists to the Biblical baby-killer King Herod - said voters were staring into the "abyss of a Labour government aided and abetted by the Scottish Nationalists or rather, not aided and abetted, but manipulated by a group of hardened political operatives".

He claimed one of the consequences of Labour's collapse in Scotland was a "Labour-Scots Nats' condominium, coalition, alliance, deal whatever you want to call it, in which you have the Scots Nats' tail wagging the Labour dog and holding this country to ransom".

The Conservative leadership's strategy has of late come under fire from a number of leading Tory figures.

Ex-Scottish Secretary Lord Forsyth has accused Mr Cameron of playing a "short-term and dangerous" game by using the SNP to damage Labour while Lord Tebbit, the party's ex-chairman, warned Tory high command's savage attacks on the Nationalists could drive Conservative supporters in Scotland into the arms of Labour as "the lesser of two evils". Indeed, Sir Malcolm Rifkind, the former Foreign Secretary, even urged Tories in Scotland to consider voting Labour to keep out the SNP.

But Mr Johnson, who is seeking to win the safe Conservative seat of Uxbridge in north London to make a return to the House of Commons, defended the Tory campaign and said: "If Nicola Sturgeon and Alex Salmond were wearing the tartan trousers, as it were, in government - Scottish tail wagging the English dog - that will build up such a sense of impatience in the English electorate that there is a real risk of a rupture."

The Mayor, who has been critical of Scottish Labour leader Jim Murphy's boast that his party's mansion tax would transfer funds from London's rich to pay for more nurses in Scotland, again hit out, saying: "There is a chop-smacking relish with which the SNP and Labour in Scotland approach the idea of taxing the Sassenachs in London and the south-east in order to pay for things in Scotland."

Mr Johnson claimed there would be a "lot of really crackers policies on defence and so on" if the SNP held the balance of power over a minority Labour administration.

"The problem with the SNP is not just that they are not wholly devoted to the health of our beloved United Kingdom, the problem is they are much more left-wing even than Ed Miliband. They want to grab the steering wheel of politics and tilt it sharply to the Left. They want to borrow another £148bn, they want to scrap Trident," he added.