Trustees of the Caledonian MacBrayne pension scheme have expressed concern over plans to put the company's ferry routes out to tender.
CalMac routes could be put out to tender
The Scottish Executive has said the routes may need to be tendered to comply with European competition rules.
But the Scottish National Party said ministers had been unable to reassure trustees over the fund and workers were afraid their pensions were in jeopardy.
The transport minister accused the SNP of "irresponsible scaremongering".
In a written reply to MSP Fergus Ewing of the SNP, CalMac managing director Lawrie Sinclair acknowledged "problems" facing the pension fund's future in the event of the ferry operator losing out to a private company.
But ministers insisted members of the pension scheme would not suffer.
Transport Minister Tavish Scott said officials were working to "try to ensure that the pension provisions were protected".
But Mr Ewing said the situation was a shambles.
He told BBC Radio's Good Morning Scotland programme: "There are nearly 1,000 CalMac pensioners and their pension fund has a black hole of nearly £25m.
"What happens to that if CalMac lose the tender? Will these workers and existing pensioners lose out on some of their pension rights, and what happens with the new company?
"I understand from the trustees of the pension fund that the Scottish Executive have brought forward no plans, no proposals at all."
The SNP MSP said ministers had not explained how they could give assurances to members, and question marks remained over the scheme's £24m deficit, which the minister claimed was only £6m.
BBC Scotland political correspondent Kit Fraser said: "My understanding is that if the scheme had to be wound up with the takeover of a new private operator, the deficit would be close to Mr Ewing's figure of £24m.
"I suppose the question is whether trying to protect the provisions and being committed to protecting them is reassurance enough for the 800 or so workers and pensioners on the current scheme."