Devonport Dockyard, which maintains, upgrades and fuels the Royal Navy's submarines, is shedding 600 jobs.
Babcock Marine, which owns the Plymouth dockyard, confirmed that it will axe the posts in the next financial year.
The company, which employs about 4,300 people at the site, bought the yard from Devonport Management Limited (DML) for £350m last year.
The Ministry of Defence (MoD) said Devonport was a "vital element" of the UK's submarine programme.
Devonport has been bracing itself for job losses since 2005 when DML predicted a downturn in submarine work.
It has suffered a series of job cuts since 1987 when it employed 13,000 people.
The yard's work has been particularly affected by the decommissioning of Trafalgar class submarines.
DEVONPORT DOCKYARD
Jack Dromey, deputy general secretary of Unite, said: "This is a body blow for a city which has already suffered cuts in defence work."
He added that the management's figure of 600 job losses had not been justified.
"Unite and the other trade unions at Devonport will go through management's plans with a fine tooth comb.
"I can assure our workforce at Babcock, and the hard-pressed people of Plymouth, that not a single job will be surrendered on the altar of redundancy without full negotiation of the circumstances," Mr Dromey said.
Del Northcott of the Prospect union which represents engineers, managers and administration staff at the yard, called on Babcock to cut the number of contractors at the yard, which is about 500.
"We need to get rid of all the contractors first and our staff should be retrained to do those specialist roles.
"Then hopefully the number of redundancies can be reduced."
The MoD said there would be £1bn of work at Devonport over the next 10 years.
A spokesman said: "The MoD will continue to work closely with Babcock Marine to devise an optimum programme of submarine and surface ship work in order to maintain a valuable skills base, although we understand the company needs to reduce its cost base in line with the reduction in submarine workload."
'Fast-changing'
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.
Its main role now is refitting Trident submarines, making it reliant on the future of the nuclear deterrent.
But Plymouth MP Linda Gilroy said the yard faced a bright future if it could continue to diversify.
Last year DML signed a £30m contract with the Ministry of Defence to build new patrol vehicles.
She said: "There is a lot we can do to make sure we don't lose quality jobs at the yard.
"We are living in a fast-changing world and we are in a very good position to benefit."
Workers were told they will all get a share of last year's profits, equivalent to a £450 bonus.
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