Scotland will definitely become independent one day, Nicola Sturgeon vowed today as she again refused to rule out a second referendum on breaking up the Union. The SNP leader said the 'direction of travel' was towards independence, as she set out her demands for her to support Ed Miliband if Labour falls short of a majority. But David Cameron warned of the 'frightening' prospect of the SNP holding Labour to ransom and demanding they ditch new roads and hospitals in England, Wales and Northern Ireland. Scroll down for video . SNP leader Nicola Sturgeon said the 'direction of travel' was towards independence butPrime Minister David Cameron warned nationalists would use a Labour government to break up the Union and punish other nations . The Tories have unveiled a new poster showing Nicola Sturgeon pulling Ed Miliband's strings, which could mean more taxes and borrowing, while England, Wales and Northern ireland suffer . David Cameron has urged people to vote tactically, to keep a Labour-SNP government out of power. The Prime Minister issued a plea to Ukip and Lib Dem voters to lend their support to the Tories to prevent 'Ed Miliband, propped up by Nicola Sturgeon demanding ever more borrowing and more taxes'. Writing in the Sunday Times, the Tory leader said: 'A vote for any of the other parties makes it harder for Conservatives to get those 23 constituencies we need to form a majority government. 'Vote for Ukip or the Liberal Democrats and you take Ed Miliband and Nicola Sturgeon a step closer to Downing Street — and our government, economy and country a step closer to chaos. 'So if you are considering voting Ukip or Lib Dem, I urge you to think of the chaos of a weak Ed Miliband, propped up by Nicola Sturgeon demanding ever more borrowing and more taxes. Only a Conservative vote in your local constituency will keep Ed Miliband and the SNP out and secure Britain's future.' Mr Cameron warned that with the SNP calling the shots, the rest of the United Kingdom – England, Wales, Northern Ireland, 'wouldn't get a look-in'. Both Mr Miliband and Ms Sturgeon have ruled out a formal coalition, with SNP ministers sitting in a Labour Cabinet. But Ms Sturgeon has repeatedly offered to work with Labour to 'lock' the Tories out of power. In September's referendum, 55 per cent of people voted against independence, but since then support for the SNP has soared. Latest polls suggest more than half of Scots plan to vote for the SNP, robbing Labour of dozens of seats north of the border. On BBC One's Andrew Marr Show, Ms Sturgeon would only say: 'I have no plans for another referendum at this stage.' But she added: 'I think Scotland will become an independent country one day. I think that's the direction of travel. 'Whether there is another referendum, and whether Scotland becomes independent is not a matter for me to decide. It is a matter for the Scottish people to decide.' Ms Sturgeon tried to play down the idea that the SNP would attempt to cause disruption at Westminster to further its goal of independence. 'I've got to persuade people that the SNP in the House of Commons after the election will not be any kind of disruptive force. We want to be constructive, to get better politics coming out of the Westminster system. 'As long as Scotland remains part of the Westminster system, it matters to people that we get good decisions out of that system, and I want the SNP to play a positive part in improving politics at Westminster for people right across the UK.' But Mr Cameron warned that if Labour is forced to rely on the SNP to pass laws and budgets, England, Wales and Northern Ireland will pay the price. Ms Sturgeon tried to play down the idea that the SNP would attempt to cause disruption at Westminster to further its goal of independence . Mr Cameron warned that with the SNP calling ths shots, the rest of the United Kingdom – England, Wales, Northern Ireland, wouldn't get a look-in . The Prime Minister, who described himself as a 'fundamental believer in the United Kingdom', told Andrew Marr: 'This would be the first time on our history that a group of nationalists from one part of our country would be involved in altering the direction of the government of our country. I think that is a frightening prospect. 'People thinking in their own constituencies, is that bypass going to be built, will my hospital get the money it needs? Frankly this is a group of people that wouldn't care about what happened in the rest of the country. 'The rest of the United Kingdom – England, Wales, Northern Ireland, wouldn't get a look-in. 'That is the prospect we face if we don't get the majority Conservative government that is in our reach.' A recent TNS survey showed the SNP has almost doubled its lead over Labour in a month, with 52 per cent now backing Ms Sturgeon with only 24 per cent of Scots planning to vote for Mr Miliband . Last week Labour announced plans for a freeze in rail fares, paid for by scrapping road upgrades on the A27 along the south coast and the A358 in Somerset - both in Tory and Lib Dem seats. Mr Cameron said: 'The SNP is a party that doesn't want to come to Westminster to contribute to a government, it wants to come to Westminster to break up our country. 'You have to ask yourself, if you are in a voter in England, in Wales or in Northern Ireland, would these people care at all about what happens in my life or my constituency. And the answer is no. 'We have already seen Labour cancelling important infrastructure projects, in the south of England, in the South West of England, the A27… . 'They've said we wouldn't go ahead with those, they've cancelled, they've said we don't care about these things because they don't matter to us politically. 'Imagine what it would be like with SNP MPs.'
SNP leader says she is not planning second referendum 'at this stage' Forced to deny her MPs will wreak havoc in Westminster after the election . Cameron warns Labour already punishing areas where they have no seats . SNP holding Labour to ransom means rest of UK 'wouldn't get a look in' Cameron urges tactical voting from Ukip and Lib Dems to block Labour .