Boris Johnson last night urged Ukip voters to 'swing behind the Conservatives' to avoid the 'nightmare' of a 'backward-looking Labour government' propped up by an 'even more Left-wing' SNP. The London Mayor claimed Ukip supporters are in 'increasing psychological conflict' as they realise a vote for Nigel Farage's party is a 'vote for Ed Miliband'. 'A lot want to come home to the Tories. Any gain by Ukip is a gain by Miliband,' he told the Daily Mail as he campaigned in the Thanet South, the Kent seat where Mr Farage is standing. 'The risk is that would let in a Labour/SNP government which would be a catastrophe for the country'. Scroll down for video . Boris Johnson receives a kiss on the cheek while campaigning in Finchley High Road and Edgware High Street . Mr Johnson poses for a photo with a group of locals while out on the Tory campaign trail . Two young women take a photo with Mr Johnson while he campaigned on the streets of London yesterday . Mr Johnson also visited Ramsgate, Kent, to support local candidate Craig McKinley . While in Kent, the Mayor of London warned that a vote for Ukip would be equal to a vote for Ed Miliband . Mr Johnson's intervention shows the Tories are determined to ramp up their warnings over a Labour government propped up by the SNP . He also urged Labour and Liberal Democrat voters to rally behind the Conservatives in Thanet and defeat the Ukip leader, an outcome he said would mean Ukip's 'puffball had exploded'. Mr Johnson's intervention signals that the Tories are determined to ramp up their warnings over a minority Labour government dependent on the votes of SNP MPs. Yesterday, former prime minister Sir John Major clashed with Scotland's First Minister Nicola Sturgeon over the prospect of the surging Scottish Nationalists, on course to win 40 or more seats, holding the balance of power at Westminster. Miss Sturgeon has ruled out any deal with the Conservatives and said she would be prepared to put Mr Miliband in Number Ten even if Labour wins fewer seats in a hung Parliament. In an unusual intervention in contemporary politics, Sir John warned the SNP represents a 'real and present danger' to the future of the United Kingdom. A partnership between Labour and the SNP would mean Mr Miliband being subjected to 'a daily dose of political blackmail' from an SNP determined to 'create merry hell' in order to bring about the end of the Union, he said. His remarks drew a furious response from Miss Sturgeon, who said his remarks about her party being a threat were 'an affront to democracy'. Mr Johnson leaves a walkabout in Ramsgate, Kent, with lipstick on his cheek . 'These are the same politicians that during the referendum campaign urged Scotland to lead the UK not leave the UK. Now they appear to say Scotland's voice should only be heard if we say the things they want us to say and vote the way they want us to vote,' she said. 'My message to John Major is Scotland's voice deserves to be heard in whatever way the Scottish people choose, and voting SNP means Scotland's voice will be heard more loudly and strongly at Westminster than it has ever been heard before.' Former First Minister Alex Salmond insisted Sir John's position was 'fundamentally undemocratic'. He said he was 'the man who, let us remember, rather carelessly mislaid every single Tory seat in Scotland'. 'What these Westminster parties fear - and what many people in England would welcome - is not just the size of the SNP, but the majority for progressive politics that would come from it,' he said . Mr Miliband, who has repeatedly refused to rule out relying on the support of the SNP to get into Downing Street, also reacted angrily. 'David Cameron is setting one part of part of the UK against another. That is dangerous. He is talking up the SNP chances and not taking them on. That is dangerous,' the Labour leader said. 'I have to say to Conservatives, frankly, I think they should tell the prime minister to stop because he is demeaning his office, he is demeaning himself, he is demeaning those people he sends out on his behalf, and frankly, I think it is threatening the integrity of the United Kingdom.' Even one former Tory Cabinet minister expressed unease, claiming the party's campaign attacks on the SNP risk undermining the future of the United Kingdom. Former Scottish Secretary Lord Forsyth said talking up the threat posed by the Scottish nationalists was 'short-term and dangerous'. The Prime Minister was unrepentant last night, rejecting the suggestion he risks stoking SNP support in Scotland. SNP leader Nicola Sturgeon was mocked by ex-PM Sir John Major over her demand to play a role in propping up a Labour government . Mr Miliband yesterday claimed David Cameron was 'setting up one part of the UK against another' 'I am responsible for lots of things. Heck, I am the prime minister, I am responsible for everything. But I am not responsible for the fact that the Labour Party has failed to get its message across in Scotland. 'The rest of the country does have to wonder, what is the price that they will pay? I fear for our country if this were to happen.' Mr Cameron said the SNP would want voters across the UK to feel by the end of five years of chaotic administration that 'the country doesn't work, that the government does not work, that the country should break up'. Campaigning in Pudsey, West Yorkshire, he added: 'By raising this I am not only raising the issue and the fear that people have that money will be sucked out of areas like this. 'I am also raising the point that we care about our UK, I care passionately about it. Don't let the SNP into government of the UK because they would try to break it up.' Mr Johnson, campaigning in Thanet, said: 'Ukip voters can see the argument that we have got the plan for the economy, and I hope very much that they will think sensibly and carefully. 'There is an element of internal conflict in the minds of many Ukip voters. They are thinking they could end up with something – a backward-looking Labour government propped up by an even more Left-wing SNP – that would really set the country back.' Mr Cameron has repeatedly warned against allowing the SNP into government for fear they will 'try to break it up'
London mayor Boris Johnson has urged Ukip supporters to vote Tory .
He said Ukip supporters realise a vote for Ukip is a 'vote for Ed Miliband'
Conservative votes would avoid the 'nightmare' of SNP propping up Labour .
Mr Johnson yesterday campaigned in Kent, where Nigel Farage is standing .