Defence giant BAE Systems is reviewing its warship business, raising fears of job losses at sites including its historic Portsmouth dockyard.
The GMB union said consultants had been hired as part of a study into the future of the company's shipbuilding business.
Regional organiser Gary Cook said there was only a few years' work left at Portsmouth, with the prospect of winning orders to build merchant ships "not great" because of subsidies paid to firms in other countries, particularly the Far East.
"BAE is looking for naval work from other nations. There are opportunities and the UK is very well placed because we have a reputation that is second to none, with a highly skilled and motivated workforce. This must be made to count for something," he said.
The union's comments followed speculation that Portsmouth was the likely victim of a review of the group's shipbuilding operations. The company employs around 1,500 workers at the yard, with a similar number in support roles.
A company spokeswoman said: "As part of our business planning activity, we are reviewing how best to retain the capability to deliver and support complex warships in the UK in the future."
This would be in line with its commitments, she added. "This work is ongoing and we will keep our employees and trade union representatives fully informed, as it progresses."
Ian Waddell, national officer of Unite, said: "This news will be deeply unsettling for the people of Portsmouth. It is not just the jobs of 3,000 at stake here, but also those of thousands more locally who depend on the shipyard.
"BAE must meet with the unions as a matter of urgency to discuss all solutions for saving the workforce. Equally, the Government has a role to play.
"Its cuts are ultimately the reason Portsmouth's docks are now having to fight for a future."
