Workers at the Cammell Laird shipyard on Merseyside have voted to strike in a row over pay and conditions.
The welders, pipe-fitters, electricians and others are building submarines and ships for the Royal Navy.
Around 96% of those who voted wanted to walk out, with 75% of the more than 400 union members of Unite and GMB taking part in the ballot.
Dates will be “announced in due course”, the unions said.
The unions said their members raised problems with bullying and harassment at the site. A petition claiming trade union members have been discriminated against has been signed by hundreds of members, the unions said.
They added: “There have been complaints about sexual harassment and disability discrimination.”
Unite general secretary Sharon Graham said: “Cammell Laird has signed contracts worth hundreds of millions with the UK Government to build and maintain its ships, yet it thinks it can get away with cutting our members’ pay in real terms.
“Unite won’t stand for such behaviour or any attempts by management to bully, harass or discriminate our members.”
GMB organiser Albie McGuigan said: “These are skilled workers doing vital work for the Royal Navy and the commercial sector.
“Yet because of Cammell Laird’s poor response to their pay claim and unnecessary delays they’re struggling to keep up with the cost of living. It’s no wonder they’re frustrated, angry and have voted to strike with a huge majority.”
The unions said their members are working on the Navy’s Dreadnought-class submarines, which will carry the UK’s Trident nuclear missiles.
The union members also include fitters, labourers, cleaners and office staff.
Cammell Laird was contacted for comment.