The Inchgreen Dry Dock, in Greenock, Inverclyde, has been unused for 17 years and was cited as a hub for Royal Navy aircraft repairs, plans which have since fallen through.
Bosses from Peel Group and subsidiary company Cammell Laird had promised to revitalise the Greenock facility and boost employment if a bid for a £5 million Royal Navy maintenance contract for aircraft carrier HMS Queen Elizabeth was successful.
But the UK Government has awarded it to Babcock in Rosyth, Fife.
Instead, the dry dock, which is 305m long by 45m wide, could be used to renovate ferry fleets, campaigners have said.
Inverclyde Council, in partnership with site owners Peel Ports, have gained initial approval to create a £9.4 million marine training and manufacturing facility in and around Inchgreen, although not including the dry dock itself.
Ex-shipyard worker Robert Buirds launched the Save Inchgreen group in December 2017.
He believes plans by owners Peel Ports to make the dry dock a hub for Royal Navy aircraft carrier repairs and maintenance were never likely to come to fruition because of the scale of investment needed to bring the facility up to scratch.
But Mr Buirds is optimistic the dry dock could be put to good use and close a gap in the market for the likes of Gourock-based Caledonian MacBrayne.
Mr Buirds said: “I never for a moment believed a word about it when there are already facilities up and running for the MoD so it wouldn’t make sense to have to invest heavily in a site like Inchgreen to be used only every four or five years for refit.
“The campaign is now driven towards turning the dry dock into a shipyard, not just for ship repair and marginal work.
“There’s a boom in shipbuilding and order books in Germany, France and elsewhere are quite substantial while we’ve killed off all but all of our shipbuilding companies.
“The last one standing is Ferguson’s.
“We have a ferry fleet that needs modernised.
“They’re crying out for that now.
“Operators are feeling pressure from insurance companies, they don’t want to ensure the rust buckets they currently have.
“There’s a big demand.”
Mr Buirds said the project between Inverclyde Council and Peel Ports could benefit the area, but is concerned the largest mainland dock of its kind in Britain could be left behind.
He said many Scots ex-pats who have kept an interest in their hometown have voiced support for his idea, with 3,500 signatures on a petition backing the proposals.
Mr Buirds said: “We’ve had contact from a lot of different people, folk from America, Canada, Australia.
“They tend to be those who moved away from Inverclyde and emigrated who are encouraging us to keep going because they remember when they used to work in the yards here and when they closed down they had to move away so they see the benefit of local employment.
“There’s opportunities for the new market and the old market.
“We will keep the pressure on.”