The Defence Secretary has formally dismissed Jeremy Corbyn's complaint that the head of the Armed Forces showed political bias.
Michael Fallon said General Sir Nicholas Houghton had behaved "quite properly" in expressing views on Britain's nuclear deterrent and military action against Islamic State (IS) in Syria.
In an interview on Remembrance Sunday, the Chief of the Defence Staff was asked about Mr Corbyn's insistence that there were no circumstances in which he would press the nuclear button.
"It would worry me if that thought was translated into power," Sir Nicholas told the BBC's Andrew Marr Show.
"The reason I say this - and it's not based on a personal thing at all - is purely based upon the credibility of deterrence. The whole thing of deterrence rests upon the credibility of its use.
"When people say they're never going to use the deterrent, I say you use the deterrent every second of every minute of every day - the purpose of the deterrent is you don't have to use it because you effectively deter."
In a strongly-worded statement, Mr Corbyn branded the comments a "matter of serious concern" and said he was writing to Mr Fallon to complain.
"It is essential in a democracy that the military remains political neutral at all times," he said.
"By publicly taking sides in current political arguments, Sir Nicholas Houghton has clearly breached that constitutional principle."
But in a letter to Mr Corbyn, Mr Fallon flatly rejected the criticism.
"For over 60 years governments of all parties have supported our nuclear deterrent," he wrote.
"Indeed I understand that remains your party's official policy, albeit one that you disagree with.
"In his interview CDS was, as he stated expressly, making a point about the credibility of the deterrent rather than any individual.
"As the principal military adviser to the Government, I consider it entirely proper for him to answer how we maintain the credibility of the ultimate guarantee of our security."
Mr Fallon said the Government believed there is a "strong case" for extending RAF air strikes to IS - also known as Isil or Daesh - targets in Syria.
"The slaughter of innocent people in Paris demonstrates the very real international threat posed by Isil," he said. "The French government has said those attacks were planned in Syria.
"The UK will be safer once we have defeated Isil and that means joining France and others striking at its heartland in Raqqah.
"When asked, CDS gave his military advice that the UK could offer qualitative support to coalition operations in Syria. Again I consider this to be a subject on which CDS can quite properly offer his view.
"CDS is unwavering in his commitment to the principle that members of the Armed Forces are politically neutral and serve the Queen and the government of the day. I am confident that he will continue to uphold it."
Mr Fallon added: "As you are now a member of the Privy Council, I would like to extend an invitation to meet with CDS, and the other defence chiefs, to discuss the current threats to our national security."