Outrage: SNP leader Alex Salmond, pictured, was caught boasting in secret footage . Alex Salmond has provoked outrage after he was caught boasting: 'I'm writing Labour's budget.' Secret footage showed the former first minister, who is bidding to become an MP, mocking Labour after it slapped down its Scottish leader Jim Murphy, who claimed he would write the party's budget north of the border. 'I knew that [he wouldn't be writing it] already – because I'm writing the Labour Party budget,' Mr Salmond declared to cheers at a fundraising event. He 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. It prompted David Cameron and Boris Johnson to join forces to warn of a looming 'constitutional crisis'. They questioned the legitimacy of the SNP's plan to try to put Ed Miliband in Downing Street, even if Labour wins fewer seats at the Election. Earlier in the day, the Prime Minister had tweeted the footage of Mr Salmond with the message: 'This footage will shock you: Alex Salmond laughs & boasts he'll write Labour's budget. Vote Conservative to stop it.' Mr Cameron, in his first joint interview with the London Mayor, his chief rival to the Tory leadership, 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.' 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'. 'Lots of people ... people who might even have voted Labour ... will think that this is a constitutional crisis,' he added. Mr Johnson, who is seeking to return to the Commons in the West London seat of Uxbridge and South Ruislip, also said a Labour government dependent on the SNP's support would be 'totally shambolic'. The SNP has declared it will never support the Conservatives in a hung parliament, but it 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. Mr Miliband dismissed Mr Salmond's claim in the footage as 'fantasy and nonsense'. But he again refused to rule out relying on SNP votes in a hung parliament. Alex Salmond boasted that he would write Labour's first budget because the SNP would hold the balance of power . 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 (pictured above) 'We are going to be writing the first Labour budget,' he told BBC Radio Five Live. 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. Responding to the leaked footage, Mr Salmond said Mr Cameron should 'try holding a few public meetings and meeting real people – and develop a sense of humour'. Former SNP leader Alex Salmond has boasted that 'whoever holds the balance holds the power' in Parliament . The video of Mr Salmond's remarks came after David Cameron tweeted the footage online earlier today . The Prime Minister said Labour had not ruled out a deal with the SNP and would be held to 'ransom' by Mr Salmond . '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,' Mr Salmond said. 'David Cameron is clearly a prime minister with both a people bypass and a sense of humour bypass.' It came as Nicola Sturgeon, Mr Salmond's successor, set out another policy demand for the entire UK, saying that SNP MPs would vote for a £24billion increase in NHS spending. Last night, Labour deployed former prime minister Gordon Brown, who claimed the only way Mr Cameron can win the election 'is to build resentment in Scotland of the English and resentment in England of the Scots'. However, he also warned that an SNP surge could lead to 'months of constitutional chaos'. SNP leader Nicola Sturgeon was out campaigning today in the Portobello area of Edinburgh where she posed with a sign highlighting her hope of a hung parliament . Ms Sturgeon has called for an increase in government spending of some £148billion and demanded her MPs have a say over the running of the whole of the UK, raising the prospect of Scottish nationalists calling the shots on policies which only apply south of the border. EU bank warns of Labour risk to UK jobs . By Hugo Duncan, Economics Correspondent for the Daily Mail . A Labour government propped up by the Scottish nationalists could be 'bad news for growth and jobs', one of Europe's biggest banks warned last night. Analysts at ING said that in return for helping Ed Miliband get into No.10, the SNP would push for more public spending and even another referendum on Scottish independence. According to the bank, this could dent global investors' confidence in the UK 'as questions are raised over the durability of the country as it stands'. The report also claimed that a referendum on Britain's future in the EU – as promised by the Conservatives – 'throws up concerns regarding trade'. 'Both the future of Scotland and its relationship with the rest of the UK together with the UK's future relationship with the EU are issues that are causing concern among investors,' said James Knightley, senior economist at ING. Mr Knightley said Britain has attracted more foreign direct investment than any other EU country in recent years – a crucial driver of growth. 'Political and economic uncertainty from the election means this could change, which would be bad news for growth and jobs,' he added.
Alex Salmond has been filmed mocking Labour's weakness in Scotland .
The former SNP leader said he would be in charge of Labour's first budget .
David Cameron said the footage, which he tweeted, would 'shock' voters .