Strike action at Cammell Laird has already cost the Birkenhead shipbuilder more than £1.5m in lost business, the company’s chief executive said.
John Syvret warned “more losses are imminent” as union bosses pledged to continue strike action in the new year.
Members of the Unite union began industrial action, including a ban on overtime, last month over management’s “callous” plans to cut 291 jobs.
Those strikes will now be extended from mid-December until mid-January.
Unite fears workers could lose their jobs the week before Christmas.
But Mr Syvret said: “This counterproductive strike action only undermines our ability to create jobs and work.
“This strike action has facilitated a loss of over £1.5m worth of business already, and with growing concerns from other customers more losses are imminent.
“It may be obvious but put simply, without customers there are no jobs at all. These lost contracts are now being undertaken in other shipyards.”
Unite assistant general secretary Steve Turner said the government should stop “sitting on its hands” as shipyards across the UK face a “growing crisis”.
“We are demanding that the government steps up and brings forward already committed work from the Royal Navy while we work with other key customers to bring projects scheduled for later in 2019 into the yard early.
“There is no reason for these job losses and it is time Cammell Laird started working constructively with Unite, and for everyone with an interest in a strong future for UK shipbuilding, to come together to protect vital skills and jobs before they are lost forever.”