The engineering group, Babcock, has announced that 290 jobs are to go at the Rosyth naval dockyard in Fife.
The dockyard carries out refits on navy vessels
The firm said the cuts were necessary as a result of the continuing drive to reduce costs and increase efficiency.
Babcock said the restructuring had played an essential part in winning recent contracts to refit warships.
It is hoped the cuts will be made through voluntary redundancies. Unions were aware of the cuts but had hoped the job losses would be lower.
Amicus official Hugh Scullion said there was a 90-day consultation period which they hoped to use to reduce the number of redundancies.
Workers at Rosyth were not competing on a level playing field with those at Devonport in the south west of England and were being treated unfairly, he added.
The losses will leave the number of workers at the yard at just over 1,500.
Chief executive Peter Rogers said: "The restructuring of Rosyth into four separate business units is now virtually complete and has delivered demonstrable benefits, both in aligning the cost base and in developing commercial opportunities such as the substantial work on Heathrow Terminal 5."
Rosyth was chosen for the Heathrow project as it has the capacity to manufacture structures on the large scale required. These would then be shipped to the construction site west of London.
Mr Rogers continued: "Winning this work in the face of stiff competition is excellent news for Rosyth and demonstrates that the continuing drive to reduce costs and increase efficiency can result in contract wins."
A company spokesman said the Ministry of Defence had agreed to cover the bulk of the cost of the redundancies as part of a deal signed when Babcock bought Rosyth from it in the mid-1990s.
Earlier this week, it was announced that Rosyth had won contracts to refit HMS Edinburgh and HMS Walney.
Jack Dromey, deputy general secretary of the TGWU, said: "This is a sad day at the end of what promised to be a good week for Rosyth.
"We fought and won new work for the dockyard. It is wrong that the company should then reduce the workforce, because that will mean too many contractors and too few directly employed workers.
Heathrow's Terminal 5 is taking shape
"The unions will fight for every job and resist any moves for compulsory redundancy."
Tricia Marwick, SNP MSP for Mid Scotland and Fife, said: "Babcock and Rosyth is a big employer and naturally any redundancies are going to affect the wider community.
"I am very disappointed indeed and it's down to the uncertainties about getting orders and it's extremely disappointing for the workforce.
"I think decisions on orders could have been quicker - there is one that has now been delayed till after 2005.
"Perhaps a bit more certainty about where the orders are going might have in the short term provided cover at the yard and would have allowed Babcock to keep these people on."