The owners of a north Devon shipyard have told workers receivers will take over the yard on Tuesday morning.
Workers at the shipyard have been on reduced hours for 15 weeks
Up to 700 people lined the entrance of Appledore Shipbuilders on Monday, barricading gates and blocking the main entrance with a digger.
Residents from the nearby village of Appledore joined the protest by demonstrating outside the yard.
At least 550 jobs are at risk if the firm, England's oldest commercial ship builder, closes.
Hundreds of others, who work for contractors, would also be affected.
One worker, Andrew Atkinson took his two young sons to the sit-in.
He said even they understand what closure of the shipyard would mean.
"If the place shuts our family's life is going to change.
"Money will be tight and there won't be a Christmas this year."
The firm has been on short-time working for the last 15 weeks and its last order was completed almost a month ago.
"This has confirmed our worst fears," said Gary Smith, south west organiser of the GMB union.
"We will be seeking an urgent meeting with the receivers to see what can be done to save jobs.
"This is a symbolic dispute in terms of manufacturing because it is a cut too far for this industry.
"We believe the yard has a viable future."
Appledore Shipbuilders was founded in 1855 and the firm had one of the biggest enclosed ship-building factories in Britain in the 1960s, building hundreds of vessels.
The yard, based on the River Torridge, has built more than 350 ships, including naval fishery protection vessels, trawlers, passenger-vehicle ferries, dredgers and bulk carriers.