David Cameron has challenged Labour to rule out a "grubby" coalition deal with the Scottish National Party (SNP) amid concerns about the renewal of the Trident nuclear deterrent.

SNP leader Nicola Sturgeon has made the scrapping of Trident a "red line" in any coalition negotiations and as speculation continues that Labour may do a deal with the party the PM called for a clear pledge to renew it.

Mr Cameron said he was concerned about reports that three quarters of Labour's general election candidates oppose renewing Trident.

He also characterised a potential coalition between the parties as an alliance "between the people who want to break up the United Kingdom and the people who want to bankrupt the United Kingdom".

The Prime Minister spoke after Tory former defence secretary Liam Fox warned that Ed Miliband could do a "shabby deal" with the SNP and scrap the nuclear deterrent "simply to get the keys to No 10".

During Prime Minister's Questions, Mr Fox said: "In a dangerous world experience as a statesman is one of the many advantages you have over any alternative prime minister.

"Will you agree that in such a dangerous world the ultimate guarantee of our security is our nuclear deterrent?

"And will you confirm that he would never be involved in any shabby deal to give away our nuclear deterrent as part of a deal with the unilateralist party simply to get the keys to No 10?"

Mr Cameron replied: "You are absolutely right, the ultimate guarantor of Britain's security is our independent nuclear deterrent and that is why we support it and will make sure it is properly renewed beginning in the next Parliament.

"I think it's important that everyone in this House makes that clear pledge.

"But it is concerning that almost three quarters of Labour candidates oppose the renewal of Trident and I think now is the time for Labour to rule out any agreement with the SNP.

"Because no one wants to see some grubby deal between the people who want to break up the United Kingdom and the people who want to bankrupt the United Kingdom."