The head of the armed forces has said he would "worry" if Labour leader Jeremy Corbyn became prime minister with a commitment never to use Britain's Trident nuclear deterrent.
General Sir Nicholas Houghton, the Chief of the Defence Staff, warned that a premier who had made clear - as Mr Corbyn has done - there were no circumstances in which they would press the nuclear button would undermine the credibility of the deterrent.
"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.
"Most of the politicians I know understand that and I think, dare I say, the responsibility of power is probably quite a sobering thing and you come to a realisation 'I understand how this thing works'."
Shadow defence secretary Maria Eagle - who has previously described Mr Corbyn's comments as "unhelpful" - appeared to back General Houghton.
"I understand the point that he is making. It is the point that I made myself when Jeremy said what he said," she told The Andrew Marr Show.
She went on to defend Gen Houghton's right to speak out on such a politically sensitive issue.
"I don't think there is anything wrong with him expressing himself in those terms," she said.
Ms Eagle - who unlike Mr Corbyn is opposed to unilateral nuclear disarmament - confirmed the Labour leader had not even met her to discuss the review of the party's defence policy which she was currently conducting.
She indicated that she could resign if there was any change to the party's current position in favour of renewing the Trident submarine fleet.
"I am not a unilateral nuclear disarmer. I don't believe that that works. I think I would find it difficult (to continue in the shadow cabinet) but we are not there yet. We have got a big process to go through," she said.
Her comments again underlined the deep divisions between the party leader and a large swathe of his MPs.
They come just a week after Mr Corbyn hailed a vote by the Scottish Labour Party conference in favour of scrapping Trident.