David Cameron has said that Labour will not recover in Scotland unless the party rules out any deal with the SNP.

The Prime Minister said in an interview with the Scottish Daily Mail that the Scottish Conservatives are the only party recovering north of the border because they have said they will not deal with the nationalists.

However he said he worries that Labour leader Ed Miliband is "not strong enough" to stand up to former SNP leader Alex Salmond and his successor, Nicola Sturgeon, and their "transitional demands".

Earlier this month, Mr Miliband ruled out a formal coalition with the SNP following a Tory campaign warning that he was planning to form a government after the election with the help of the party, but he failed to rule out an informal agreement.

Mr Cameron told the Scottish Daily Mail: "Labour ought to rule out not just coalition but any form of deal with the SNP. They can't recover in Scotland unless they actually say 'We're not going to deal with these people'.

"While they are saying effectively they're going to deal with them, that's giving people in Scotland a green light to vote SNP. They've got to have the courage of their convictions.

"The only party that's actually recovering a little bit in Scotland is the Conservatives, because we've said 'We're not dealing with these people, they want to break up our country, we're not having anything to do with them. Vote for what you believe in.'

"What is Labour's message? 'We don't like the SNP but we might do a deal with them, they might help us over the threshold'. That's useless."

He said the SNP was "just trying to take the Labour Party hostage".

The battle to win at Westminster officially began yesterday as parties hit the campaign trail.

Polls continue to suggest the SNP could win dozens of Scotland's 59 seats and hold the balance of power at Westminster.

At the campaign launch in Edinburgh yesterday, Scottish Tory leader Ruth Davidson said people who voted No in the independence referendum deserved to know Labour's intentions.

Mr Cameron told the paper: "There's a difference between the SNP and other parties: they want to break up our country.

"When you're dealing with separatists you've got to remember that separation comes before everything.

"It's not austerity they're really worried about, it's not even Trident they're worried about. It's breaking up Britain they're worried about."

Speaking on BBC Radio's Good Morning Scotland programme, Ms Sturgeon said another referendum on independence would not form part of any deal with Labour.

She said: "The General Election is not about independence, it's about giving Scotland a louder voice in Westminster, making sure that Scotland's interests are protected and that the issues that matter to us are up the agenda.

"We're not going to get a referendum as a result of this Westminster election and that's not one of the issues that we would be seeking to secure.

"I think we will have another referendum and I think Scotland will become independent, but that's only going to happen after we have a party, presumably the SNP, putting another referendum in a manifesto for a Scottish Parliament election and people voting for that.

"I can't impose a referendum on the Scottish people against their will."

On whether the party's manifesto for next year's Holyrood election would include that pledge, she said: "I'll take that decision when we come to write our 2016 manifesto."

Ms Sturgeon insisted that Trident remained a "red line" issue for the SNP in any post-election deal with Labour.

She said: "Under no circumstances would we ever vote for the renewal of Trident or the spending of money on the renewal of Trident.

"In terms of any formal arrangement with Labour, I've made clear and I can't make clearer Trident is a red line."