Up to 800 shipyard jobs on Glasgow’s River Clyde could be axed by BAE Systems.
Unions have accused the government of an act of “betrayal” after previously saying jobs at the yards were secure.
The Govan and Scotstoun yards were promised a contract for 13 frigates in 2014 as part of the blackmail for workers in Scotland to vote against independence.
But that was cut to eight frigates last year. The first was set to be built this year but could now be delayed until next.
The announcement brought into sharp focus the debate at the STUC conference in Dundee last week over defence jobs diversification.
Some unions back Trident renewal, claiming it would create jobs.
Instead they must launch an all-out fight at the shipyards and call for support across the trade union movement.
Scottish unions reject Trident
The Scottish TUC conference last week reaffirmed its solidarity with refugees and campaigning against racism.
Delegates backed the General Council’s position of remaining in the EU, despite opposition from the left.
A motion backing Trident renewal was roundly defeated.
Delegates instead voted to oppose Trident and develop a strategy for retraining workers employed on Trident.