Former head of the Royal Navy Lord West of Spithead has threatened to quit Labour if the party adopts Jeremy Corbyn's policy on nuclear disarmament.
The former security minister said he would find it "probably impossible" to remain a Labour peer if the party adopted what was "effectively a pacifist policy".
A row over the Labour leader's stance on the UK's "weapons of mass destruction" overshadowed the final day of the party's conference in Brighton.
Labour is reviewing its defence policy, but Mr Corbyn said he has a mandate from his landslide leadership election win to push for his position on nuclear disarmament.
But he has acknowledged he would have to "live with" the party's policy if it opted to support the renewal of the Trident nuclear weapons system.
Lord West told the Sun on Sunday: "I personally believe we should keep the (nuclear) deterrent, but it's much more than just that - it's whole issue of defence across the board.
"If finally there is a decision within the Labour Party that a defence policy is effectively a pacifist policy I would find it probably impossible to remain sitting on the Labour benches.
"Because I don't believe that being a pacifist - although it's an admirable thing for an individual - I don't believe it's a way for someone to look after our nation because we are in a very, very dangerous and nasty world."
Lord West said he hoped Mr Corbyn would discuss the issue with him.
"I will certainly, when the House is back, make people aware that I'm very happy to discuss defence, as I have always been with people within the Labour Party and indeed previously they did consult me and ask my view.
"So I would hope Mr Corbyn would do the same."