Jeremy Corbyn is locked in a constitutional stand-off with the military after the head of the armed forces warned the credibility of Britain's Trident nuclear deterrent would be undermined if he ever became prime minister.
The Labour leader said the comments by General Sir Nicholas Houghton, the Chief of the Defence Staff, were a clear breach of the principle that the military did not intervene in issues of political dispute.
He demanded Defence Secretary Michael Fallon intervene to ensure "the neutrality of the armed forces is upheld".
Earlier however, Labour's shadow defence secretary Maria Eagle said she saw nothing wrong with Gen Houghton's comments and echoed his concerns about Mr Corbyn's position on Trident.
Her comments underline the deep divisions within the Labour ranks, with many MPs bitterly opposed to the leader's stated aim to overturn the party's current policy and commit it to scrapping the deterrent.
The latest row erupted after Gen Houghton expressed concern that Mr Corbyn had declared that there were no circumstances in which he would press the nuclear button should he become prime minister.
"It would worry me if that thought was translated into power," he told BBC1's The 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 statement, Mr Corbyn said it was a "matter of serious concern" that the Chief of the Defence Staff had intervened directly in "issues of political dispute".
"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.
"Accordingly, I am writing to the Defence Secretary to ask him to take action to ensure that the neutrality of the armed forces is upheld."
It is not the first time Mr Corbyn has clashed with the military since becoming Labour leader.
Following his election in September, an unnamed serving general was quoted by The Sunday Times as warning there could be a "mutiny" if he became prime minister.
There was no immediate comment on the latest row from the Ministry of Defence.
However, Ms Eagle - who has her herself criticised Mr Corbyn's remarks on the use of deterrent - insisted that the general was entitled to speak out on the issue.
"I don't think there is anything wrong with him expressing himself in those terms," she told The Andrew Marr Show.
"I understand the point that he is making. It is the point that I made myself when Jeremy said what he said."