Workers at Ferguson Marine "deserve certainty" after the Scottish Government rejected a request for £25m of additional investment in the nationalised shipyard.
Neil Gray, the Economy Secretary, last week refused to commit to funding a modernisation programme at Fergusons to allow it to compete for future orders.
Gary Cook, GMB Scotland senior organiser in manufacturing, said investment was crucial if the yard was to be sustainable long-term when work on the two ferries is finally finished.
Work on the Glen Sannox and Glen Rosa has been blighted by delays and soaring costs amid a political row over who is to blame.
Cook told the Record: "Whatever mistakes have been made, the workers at Ferguson Marine did not make them and whoever is to blame, it is not them.
"Their jobs and the future of this yard should not be undermined by mistakes made years ago. Whatever decisions were being taken elsewhere, in the yard, this skilled and committed workforce has done exactly what was asked of it.
"We know what happened in the past and need more than vague promises about the future.
"Ferguson Marine needs practical support and investment right now. The workers deserve some kind of certainty and the yard needs investment to win new commercial contracts and create a pipeline of secure work.”
Gray last week suggested a new business plan will be needed before more investment can be approved at the yard.
Ferguson bosses have argued the investment is needed for a new plating line for steel panels, burning tables and new computer software.
Workers suggest some of that work is needed to reinstate the yard to its set-up before tycoon Jim McColl took charge in 2014 before managers changed the layout.
GMB Scotland has called on ministers to make a direct award of a contract for seven small ferries for Calmac and say that, along with other contracts, would secure a pipeline of future work for the yard.
Cook said: "No one wants these ferries to be finished more than the workers but they also want, and deserve, to know their future will not be determined by mistakes of the past made by others.
"With a committed workforce led by ambitious managers, there is nothing to stop this yard having the brightest possible future and the Scottish Government must urgently help secure it.”
A Scottish Government spokesman said it was "committed to doing all it can to secure a sustainable future for the yard and its skilled workforce".
They added: "This includes working with the Board to build on its initial proposition and develop a new business plan and investment case.
"These must reflect deliverable market opportunities and be compliant with subsidy control rules."
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