Devonport is the oldest naval dockyard in Britain
Hundreds of job losses are expected to be announced at Devonport Dockyard in Plymouth.
The yard's owner, Babcock Marine, is due to make an official announcement on Friday, once workers have been briefed by management.
Union representatives met with company bosses earlier.
Babcock had warned previously that job losses were inevitable because of a reduction in the amount of work being done at the site.
Babcock Marine said up to 300 jobs could go at Devonport, which maintains, upgrades and fuels the Royal Navy's submarines and other vessels.
In February the company had predicted as many as 600 jobs could be lost out of a workforce of about 4,300.
It hopes most of the actual job losses can be achieved by voluntary redundancy.
Mike Carey, who is a dockyard worker at Devonport, said: "We thought it would be 600 jobs.
"We were told at one time it was going to be 800, so if it is 300 then it's probably better than expected."
Del Northcott, from the Prospect union, said: "I think it's still a disappointment.
"Not six months ago the Armed Forces Minister told us categorically there would be job gains at Devonport, not job losses."
Babcock Marine bought the dockyard from Devonport Management Limited (DML) for £350m last year.
The yard has been bracing itself for job losses since 2005 when DML predicted a downturn in submarine work.
This is the latest in a series of cuts since 1987, when 13,000 people were employed at the site.
Work has been particularly affected by the decommissioning of Trafalgar class submarines.
Devonport's main role now is refitting Trident submarines and in February the MoD said there would be £1bn of work to be done there over the next 10 years.
Devonport's origins date back to 1691, when William of Orange commissioned the building of a new dockyard to support the Royal Navy in the Western Approaches.
It is the oldest naval dockyard in Britain and the biggest employer in Plymouth.