Ed Miliband has hit back at the Defence Secretary for a personal attack in which Michael Fallon accused the Labour leader of being a "backstabber".
The Labour leader said Mr Fallon was a "decent man" but he had "demeaned himself and he has demeaned his office" with his comments over Mr Miliband's relationship with his brother.
The Defence Secretary had warned in an article he wrote for The Times that Mr Miliband would do a deal with the SNP over the Trident nuclear deterrent because he could not be trusted.
He said Mr Miliband had "stabbed his own brother in the back" to become Labour leader and would be willing to "stab the United Kingdom in the back" to become prime minister.
The two men were speaking at separate events in London.
Mr Miliband said: "Michael Fallon is a decent man but today I think he has demeaned himself and he has demeaned his office and national security is too important to play politics with.
"I will never compromise our national security, I will never negotiate away our national security and you know what, the Conservative Party can throw what they like at me but I am going to carry on concentrating on the issues that matter to the British people."
Mr Fallon made his comments ahead of today's event in which he pledged the Tories would maintain a continuous at-sea nuclear deterrent by building a fleet of four Successor Ballistic Missile Submarines if they win on 7 May.
He warned that Mr Miliband must not be allowed to trade away Britain's nuclear deterrent for a deal with the SNP that would put him in power.
Challenged on Sky News over his attack on Mr Miliband, Mr Fallon said: "We're not slinging mud. This is an issue of trust and character.
"We've made it clear we are going to renew those submarines. If you vote Labour you could end up, because of the SNP position, with a coalition where Ed Miliband does some grubby deal behind the scenes with Nicola Sturgeon of the SNP.
"We need to get that out into the open so people know, in four weeks' time, exactly what they are voting for."
Asked if he supported Mr Fallon's comments, David Cameron said: "Of course. Michael Fallon is making a very important point which is first of all only the Conservatives are absolutely guaranteeing a full replacement of Trident with four submarines and continuous at sea deterrence.
"It is important that in a dangerous insecure world we have that ultimate insurance policy."
