David Cameron has challenged Labour leader Ed Miliband to rule out any "grubby deal" with the SNP after May's General Election.
The Prime Minister said that a Labour-SNP alliance would be between "the people who want to break-up the United Kingdom and the people who want to bankrupt the United Kingdom".
And he described as "concerning" reports which suggest that three-quarters of Labour's Westminster candidates oppose the renewal of Trident.
The SNP has made scrapping Trident a key requirement of any post-election deal.
At Prime Minister's Questions former Conservative defence secretary Liam Fox claimed Mr Miliband could do a "shabby deal" with the SNP on the nuclear deterrent "simply to get the keys to No 10".
He asked Mr Cameron: "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?"
The Tory leader 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."
He want on: "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."
Some senior Labour MPs are understood to also be pressing Mr Miliband to rule out a deal with the SNP before polling day on May 7.
Angus Robertson, the SNP's Westminster leader, said: "No sensible UK Government would develop a nuclear weapons capability now if one didn't exist.
"Therefore, a strong group of SNP MPs in a hung parliament can help bring sanity to bear now - and halt Trident renewal in its tracks."
Earlier this week Mr Cameron claimed that a Labour-SNP alliance would be the "worst" outcome in May.
Senior Tories are currently weighing up their own coalition options if there is a hung parliament in May, as polls predict.
Some Conservative MPs are keen to do a deal with Northern Ireland's Democratic Unionist Party (DUP) if they fall just a handful of seats shy of an overall majority in May.
Tory chairman Grant Shapps has ruled out a post-election deal with Nigel Farage's eurosceptic Ukip.
However, Mr Cameron has not repeated that commitment.
Many in his party are also urging Mr Cameron to head a minority Tory administration, following five difficult years of coalition government with the Liberal Democrats.
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