David Cameron and Boris Johnson warned of a looming 'constitutional crisis' last night after Scotland's former First Minister Alex Salmond was caught boasting: 'I'm writing Labour's budget.' The two most high-profile Conservatives joined forces to question the legitimacy of the SNP's plan to try to put Ed Miliband in Downing Street, even if Labour wins fewer seats at the general election. The London Mayor, deployed to inject more passion into the Tory campaign, claimed it would mean 'truckfuls of taxpayers' dosh growling up the M1 to Scotland'. Boris Johnson and David Cameron joined forces yesterday to slam the SNP plans to prop up Labour . 'Lots of people… people who might even have voted Labour - people will think that this is a constitutional crisis,' Mr Johnson said. The SNP has declared it will never support the Conservatives in a hung Parliament, but is ready to do a deal with Labour, even if it finishes in second place. It would be the first time since 1924 that a second-placed party had formed a government. Last night, secret footage of Mr Salmond yesterday raised new fears about Scottish Nationalists holding the balance of power at Westminster after May 7. At a fundraising event, Mr Salmond, who is bidding to become an MP, is heard mocking Labour after it slapped down Scottish leader Jim Murphy's claim that he would write a Labour budget north of the border. 'I knew that already - because I'm writing the Labour Party budget,' he declared, to cheers and applause. Mr Salmond also joked that he would 'check his top pocket' – a reference to a Conservative election poster showing a miniature Mr Miliband in his breast pocket. Alex Salmond boasted that he would write Labour's first budget because the SNP would hold the balance of power . The Prime Minister, in his first joint interview with the London Mayor, his chief leadership rival, said: 'In the last 48 hours the SNP have said "no money spent on defence projects unless you get rid of Trident. No HS2 unless it starts in Scotland". 'You have now got Salmond saying he wants to write Ed Miliband's budget. That's before this has even happened. Imagine how much worse it would be, imagine how many pockets would be picked if this were to happen. 'The stakes at this election have always been high. It's about sticking with a plan that's working or risking it with Ed Miliband. But frankly the stakes have got much higher because of what's happening in Scotland with the SNP potentially wiping out Labour. 'The way to avoid that outcome which would be unstable, bad for the economy, bad for jobs, bad for living standards, bad for this country's prospects, the way to avoid it is to vote Conservative. 'What we have got to do in the remaining days of this campaign is bring together these arguments and show that there's an absolute link between the danger of the SNP, a weak Ed Miliband, the threat that he's making and the need to vote Conservative to deliver that majority.' Mr Johnson, seeking to return to the Commons in the west London seat of Uxbridge and South Ruislip, said a Labour government dependent on the SNP's support would be 'totally shambolic'. The two high-profile Tories yesterday visited a school in south west London, where they made some hand paintings with the students . The pair left the school together, where Mr Johnson claimed SNP's plans to prop up a Labour government would mean 'truckfuls of taxpayers' dosh growling up the M1 to Scotland' 'You would have Labour at the beck and call of the SNP. I care very strongly that they've got an agenda to break up the Union, but even worse in government… they would be dragging the steering wheel of a Labour government to the Left. 'From my perspective where I sit in in London you would see truckfuls of taxpayers' dosh growling up the M1 to Scotland. It would be a continual ransom by the SNP of a Labour government.' The Mayor added that the polls were 'starting to move a bit' in the Tories' favour, and claimed there would be a late swing to them as there was in the 1992 election. 'In the last week or so, I really think they're really going to move decisively in favour of a strong, positive Conservative message which is what we've been offering. The record of the government over the last five years has been absolutely outstanding and I think the choice is very clear. 'Do you want to go backwards under Miliband or forwards under the Conservatives? Loads of floating voters are going to make up their minds in our favour in the last few days.' Tomorrow, the Prime Minister and outgoing leader of the Commons William Hague will inflame the row further by launching a separate 'English manifesto' for the first time – as well as the traditional Scottish, Welsh and Northern Irish documents. Mr Miliband dismissed Mr Salmond's claim as 'fantasy and nonsense' – but again refused to rule out relying on SNP votes in a hung Parliament. Ed Miliband, pictured on the campaign trail in Ipswich yesterday, dismissed Mr Salmond's claims he would write the Labour budget as 'fantasy and nonsense' The Labour leader also insisted yesterday that he had no plans to negotiate with SNP . 'We are going to be writing the first Labour budget,' he told BBC Radio Five. The Labour leader insisted he had no plans to negotiate, adding: 'I am going to put a Labour Queen's Speech before the Commons and people will have to decide how they vote.' Mr Miliband also accused Mr Cameron of 'trying to stir up English hatred against the Scots' by highlighting the threat posed by the SNP's left-wing agenda. In a separate interview, with the Independent, the Labour leader refused to rule out seeking to enter Number Ten propped up by other parties even if his party wins fewer seats than the Conservatives. Nicola Sturgeon, Mr Salmond's successor, yesterday set out another policy demand for the entire UK – saying SNP MPs would vote for a £24 billion increase in NHS spending. Former First Minister of Scotland Alex Salmond said Mr Cameron had a 'sense of humour bypass' after the Prime Minister reacted angrily to the leaked video . Responding to the leaked footage of him, Mr Salmond said Mr Cameron should 'try holding a few public meetings and meeting real people – and develop a sense of humour'. 'The point made in a light-hearted way was that Scottish Labour leader Jim Murphy had been slapped down by his party bosses at Westminster and told that he would have no role in a Labour budget,' the former Scottish First Minister said. 'David Cameron is clearly a prime minister with both a people bypass and a sense of humour bypass.' But Scottish Conservative deputy leader Jackson Carlaw said: 'Here we have the SNP-Labour offer in a nutshell: a chaotic pact with Britain's future security treated like a dinner-party joke. 'After Nicola Sturgeon's slick con-trick earlier this week, this video shows the SNP's arrogance laid bare. 'The only way to prevent Alex Salmond from trying to tear up Britain by holding a Labour party to ransom is by voting Conservative.' Conservative chief whip Michael Gove, on a visit to West Aberdeenshire and Kincardine, rejected criticism of his party's warnings about the dangers of the SNP holding sway over a weak Labour government. 'What's putting the Union at risk is the prospect of an SNP victory in seats in Scotland, and the reason why the SNP have been doing well is the collapse in the Labour and Liberal Democrat vote. 'It's the Conservatives who are the strongest party defending the Union, the strongest bulwark against separatism. I think it's absolutely right that we should underline the risks of voting for a party that wants to break up the United Kingdom.'
David Cameron and Boris Johnson last night warned of a looming 'crisis'
They joined forces to question legitimacy of SNP's Labour-boosting plan .
Mr Johnson claimed it would mean 'truckloads of cash' moving up the M1 .
It comes as Mr Salmond was filmed joking he would write Labour's budget .