David Cameron has been forced to defend Michael Fallon after the defence secretary claimed Ed Miliband would stab Britain’s national interest in the back in the same way he did his brother to gain the Labour leadership.

The prime minister said there were serious issues at stake in the election campaign and Fallon was right to question Labour’s approach to the renewal of the Trident nuclear programme in “a pretty frank way”.

Fallon had claimed Miliband’s treatment of his brother, David, showed he was so ambitious that he would compromise national security and negotiate away Trident if it meant securing power for Labour. He claimed the party would have to abandon any plans to renew the fleet in order to secure the support of the Scottish National party in a hung parliament because the SNP has pledged to scrap nuclear weapons.

Miliband said Fallon was “a decent person but he had demeaned himself and demeaned his office”. Miliband’s campaign manager, Douglas Alexander, said the Tories were in the political gutter.



Ed Miliband responds to accusations by Michael Fallon that he would betray Britain’s interests Guardian

Miliband also criticised Nick Boles, the Conservative minister of skills and equality, who tweeted: “Ask yourself this. Who does Vladimir Putin want to see running Britain after 7th May? Answer: the man who abandoned the Syrians to their fate and the woman who wants to scrap our nuclear deterrent.”

Some Cabinet figures were uneasy with Fallon’s attack, with the foreign secretary, Phillip Hammond, four times refusing on BBC Daily Politics to repeat the back-stabbing claim, saying: “I will leave it to the public to make up their mind” whether Miliband might act in such a way.

At a Cameron Direct event in Nottinghamshire, the prime minister initially said: “We should try to be respectful in the way we conduct our political debate.”

But pressed about the personal tone of the allegations, he said: “When I listen to the Labour party, they are playing fast and loose with our security, they don’t say whether they will have three submarines or four submarines. They are saying, effectively, they are happy for Ed Miliband to get into Downing Street with the support of Nicola Sturgeon and the SNP.

“The SNP is saying it is a red line for them that Trident won’t be replaced, so here is the thing: we need an answer from Labour. Are you really committed to the Trident deterrent? Are you really going to have four submarines and can you rule out any arrangement where the SNP put you into power knowing they want more borrowing, more debt, and less secure defences without a Trident replacement? And that is why Michael Fallon is right to raise this and, yes, in a pretty frank way.”

Defence secretary Michael Fallon accuses Ed Miliband of being untrustworthy over Trident Guardian

Fallon confirmed a new Tory government would go ahead with the construction of four new Trident nuclear missile submarines to replace the existing fleet.

He told the Times: “Ed Miliband stabbed his own brother in the back to become Labour leader. Now he is willing to stab the United Kingdom in the back to become prime minister.”

Miliband retorted: “National security is too important to play politics with. I will never compromise our national security, I will never negotiate away our national security.”

Miliband then criticised the Tories’ campaigning style. “I think the British people deserve better than what the Conservative party are offering in this campaign, which is a campaign based on deceit and lies.”

Will the Tories’ attack over Trident hurt Labour? Guardian

The row broke out as Labour prepared to sign off its election manifesto at a “clause 5” meeting of the shadow cabinet with senior officials from the ruling national executive, the national policy forum and the parliamentary Labour party. The Conservatives said they were moving their manifesto launch to Tuesday, after both main parties announced their intention to publish on Monday.

The Conservatives appear determined to blend the issues of Miliband’s character, national security and the risk posed by the SNP holding the balance of power. The party will be pleased the news agenda has moved from its approach to non-domiciled taxpayers to defence.

Speaking on BBC Radio 4, Fallon said: “People cannot be sure if they vote Labour in England whether they would lose their nuclear submarines because Nicola Sturgeon from the SNP has made it crystal clear she will not support the renewal of Trident.

“This is the most important issue facing this country, the issue of whether we continue with our nuclear defence, and you have to make clear whether you are prepared to put the principle of our nuclear defence ahead of any grab for power and stitching together with a coalition.”

He defended his personal attacks, saying the issue was about trust and leadership.

Alexander said Fallon’s remarks “said a lot more about the state of the Tory election campaign than the state of our national security. This is desperate stuff from a rattled campaign that spent yesterday defending tax avoidance and has now descended to the politics of the gutter.

“Labour’s commitment to continuous at-sea nuclear deterrent was not up for negotiation. The experts say that will require four submarines, we will review that pending any technological advance. The Tories are like a wounded, cornered animal at the moment, they are lashing out.”

Sturgeon indicated in a Guardian interview last month that the SNP would not place the scrapping of Trident on the table in any negotiations after the election. The Scottish first minister said the SNP would simply vote against its replacement when the maingate vote is held in the Commons.

But in the Scottish leaders’ debate on the BBC on Wednesday night, Sturgeon said: “Is Trident a red line? Well here’s my answer: you’d better believe Trident is a red line.”