Unite has reaffirmed defending the jobs of tens of thousands of defence workers involved in the Trident nuclear submarine programme.
But the union also attacked successive governments for their failure to develop a defence diversification strategy that would safeguard defence jobs.
Ahead of next week's vote in Parliament on the renewal of the Trident weapons system, delegates at Unite's policy conference in Brighton backed an executive council statement opposing in principle the possession of nuclear weapons, but adding the union could not accept the "obliteration" of thousands of its members' jobs.
The statement said: "Unite does not and never will advocate or support any course of public policy which will put at risk jobs or communities.
"Defence diversification is not going to be taken seriously by this Government and we cannot ask our members in the affected industries to buy a pig in the poke.
"The possibility of new jobs of similar quality tomorrow will not support workers and their families and communities today.
"Until there is a government in office ready, willing and able to give cast-iron guarantees on the security of the skilled work and all employment involved, our priority must be to defend and secure our members' employment.
"Unite commits to campaigning to secure a serious government approach to defence diversification, enabling Britain to play its part in nuclear disarmament and urges the Labour Party to give the highest priority to this aspect in it considerations."
General secretary Len McCluskey said: "We reassert our broad principles regarding opposition to nuclear weapons and our strong support for defence diversification.
"But we know that we do not have a government in place that has any interest in giving effect to either of those principles. Here in 2016, a vote against building those submarines would have only one immediate effect. The destruction of jobs, workplaces and communities of thousands of skilled Unite members, represented in this room today.
"The statement also underlines our Defend our Spend campaign, to ensure that the UK's defence budget is spent on procurement here in Britain and Ireland, sustaining the jobs of our members, rather than exporting them to the USA or elsewhere."