Every family in the country will be left £350 worse off if the SNP is left calling the shots in a Labour government, George Osborne claimed today. The Chancellor warned of a 'dangerous cocktail' if Ed Miliband is at the mercy of SNP leader Nicola Sturgeon. With polls showing no party will win an overall majority, Mr Osborne claimed it would spark a 'constitutional crisis' if Scottish nationalist MPs held the balance of power in the UK government. Scroll down for video . Chancellor George Osborne warned of a 'dangerous cocktail' if Ed Miliband is at the mercy of SNP leader Nicola Sturgeon . Launching the SNP manifesto this week, Ms Sturgeon named her price for supporting Labour in any post-election deal. She 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. And she unveiled a £140billion 'ransom note' for Britain to drag Labour to the left, including an end to austerity, rising benefits payments and the abolition of Trident as the price Mr Miliband would have to pay for getting into Number 10. Mr Osborne seized on the demand for more spending and more borrowing and warned that Treasury analysis suggests it would add £6billion to Britain's interest bill, worth £350 per family. Mr Osborne told BBC Radio 4's Today programme: 'It's a new feature of this election that a Labour party cannot win an overall majority, so they would rely on this deeply unstable block of Scottish Nationalists. 'We've got a strong plan for a national recovery, but it's perfectly reasonable to point out with two weeks to go until this election the alternative is what Gordon Brown described last night as 'constitutional chaos'. 'The Scottish Nationalist leaders would clearly be the stronger force in a Labour minority government propped up by them.' Alex Salmond boasted that he would write Labour's first budget because the SNP would hold the balance of power . He said global investors warn that an SNP-Labour deal 'will lead to higher market interest rates, which means higher mortgage rates for families, which means higher bills, cut incomes, it means businesses finding it more difficult to expand'. He added: 'Let's take one of Canada's largest banks, CIBC. They say a Labour government supported by the SNP would lead to a dangerous cocktail where taxes are going up, spending is going up, interest rates are going up. 'They have been joined by Morgan Stanley, the US investment bank, BlackRock, which is the largest pension fund in the world, Deutsche Bank, the largest German banks. These are pretty unsentimental people. 'These are economists and investors who make a living predicting what is going to happen to different economies around the world. And this week they have come out in a chorus to point out the consequences for the UK of departing from the economic plan we've pursued.' The warnings come after former SNP leader Alex Salmond boasted: 'I'm writing Labour's budget.' The former Scottish First Minister, who is set to be one of up to 50 SNP MPs elected next month, was filmed at a rally mocking Labour's weakness north of the border. He highlighted comments by the shadow business secretary Chuka Umunna – who tried to downplay claims the Scotland could avoid spending cuts by saying the party's leader north of the border Jim Murphy 'will not be in charge of the UK budget'. But Mr Salmond boasted that he already knew this – because that job would fall to the SNP. He said: 'Labour's financial secretary said the Scottish Labour leader will not be writing the Labour party budget. I knew that already, because I'm writing the Labour Party budget.' Labour leader Ed Miliband today warned voters in Scotland that every MP Labour does not win north of the border makes a Tory government more likely . It prompted David Cameron and Boris Johnson to join forces to warn of a looming 'constitutional crisis'. Mr Cameron said: '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.' Mr Johnson warned it would mean 'truckfuls of taxpayers' dosh growling up the M1 to Scotland'. Mr Miliband today warned voters in Scotland that every MP Labour does not win north of the border makes a Tory government more likely. He told ITV's Lorraine: 'It's either a Labour government or a Conservative government after the election. 'And every one less Labour MP means it's more likely that David Cameron will be the Prime Minister again. People have to make their choice, but we will keep fighting that big argument about how we can create a better Scotland for working people and a better United Kingdom.' Challenged about why he is not willing to work with the SNP, he added: 'The reason why I am not going to have a coalition with the Scottish national party is we have got fundamental differences . 'And the most fundamental difference of all is they still want to break up the country. 'They have ruled out having another referendum on independence within the next five years. 'They used to say it was once in a lifetime. When you have got those big, big differences I can't see how having a coalition with them.' The Tories will tomorrow inflame the row over a potential Labour-SNP power-sharing deal by unveiling the party’s first ‘manifesto for England’. Mr Hague is expected to warn that England risks being ‘held to ransom’ by Scottish Nationalists unless Labour agrees to implement such a reform. The manifesto is also expected to commit the Tories to dozens of new road schemes in England and an extension of the Cancer Drugs Fund, which pays for expensive cancer treatments on the English NHS. Inflammatory: Prime Minister David Cameron (left) will be joined by William Hague (right) as they launch the English manifesto - with the latter expected to warn England risks being 'held to ransom' by SNP . The document will be seen as an attempt by the Tories to tap into growing English nationalism. Opponents are likely to criticise it as risking fostering division between the UK’s four nations. But a Tory strategist said: ‘We have separate Scottish, Welsh and Northern Irish manifestos. Why shouldn’t there be one for England?’ The ‘English votes’ plan would seek to resolve a longstanding constitutional question which will become more urgent than ever once the Scottish Parliament is handed yet more powers. Mr Hague wants to strip Scottish MPs of the power to impose tax changes, education and health reforms on England, given Holyrood will soon control all these issues in Scotland. The reform would make it impossible for any future Labour government to press ahead with its plans to raise the top rate of tax to 50p in England without winning the support of a majority of English MPs. Tensions: The manifesto is expected to inflame tensions between the Tories and nationalist parties like Nicola Sturgeon's SNP and Plaid Cymru, led by Leanne Wood . It could also make it difficult for a Scottish MP to again hold the office of Chancellor or Prime Minister, given they would be unable to vote on some of their own Government’s legislation. For decades, Labour has relied on the votes of its power base of Scottish MPs to rule Westminster. But since the creation of the Scottish Parliament, critics have questioned how it can be right for Scots MPs to have the crucial say on issues such as tuition fees and foundation hospitals which no longer affect their constituents. Both reforms passed under Tony Blair’s government thanks to the votes of Labour’s tartan army of MPs. Mr Hague is expected to say that the scale of devolution now to be given to Scotland means the question of the status of Scottish MPs at Westminster is unavoidable.
Chancellor warns global investors fear impact of SNP in UK government . Claims there would be a 'constitutional crisis' if nationalists call the shots . Miliband warns Scots every fewer Labour MP helps make Cameron PM .