The company said its operations centred on Mull but these could not resume without guaranteed space on ferries to the island.
The firm, run by bus and coach operator West Coast Motors, said it had sought 40 spaces per sailing on the Oban-Mull route.
CalMac said it could not allocate that number of spaces to one firm and had suggested alternatives which had been rejected by West Coast Tours.
West Coast Motors and West Coast Tours managing director Colin Craig said: “On a day when tourism businesses have been allowed to re-open after the most challenging four months in living memory, I write to advise that instead of re-starting our Mull Tours business we are instead facing the very difficult prospect of shutting it down.
"This will result in approximately 12 members of staff, mainly based on the Isle of Mull, facing the hardship of redundancy and loss of income.
“The reason for this is not due to Covid-19 but instead is down to Caledonian MacBrayne’s capacity restriction on the Isle of Mull ferry which I consider excessive and ruinous for the businesses and economy on the Isle of Mull.
“As a public transport operator in the bus and coach sector, we have operated throughout the pandemic and are acutely aware of the safety measures necessary to keep staff and customers safe at all times.”
“We engaged in positive discussions with CalMac regarding securing 40 spaces per sailing and a joint marketing campaign to promote the tours.
"On Monday, CalMac decided they could no longer honour this arrangement.
“It is deeply frustrating and disappointing to be denied the opportunity by Cal Mac to re-start our tours.
"I’ve no doubt this decision will have a negative impact on local hospitality and tourist businesses on both the island and mainland and public finance.
"The lack of capacity hits our island bus services that are being supported by Transport Scotland’s Covid support scheme.
“Whilst the Isle of Mull starts to re-open it is difficult to understand Cal Mac’s decision to limit capacity to such a low level."
However, CalMac said it could not give preferential treatment to individual firms.
Operations director Robert Morrison said: “CalMac has had to restrict space on all of our vessels as a direct result of the ongoing global pandemic.
"These moves, in line with Scottish Government guidelines, are designed to protect health and save lives.
"That is our absolute top priority.
“Our contract would not allow for us to allocate 30 per cent of available capacity of a sailing to a private company at the expense of residents and other island businesses.
"That would be unfair to all other passengers.
“We have been in constant engagement with officials from West Coast Motors over the past few weeks, including yesterday, and have suggested a number of alternatives for them which they rejected.
“We sympathise entirely with anybody that is seeing their business affected just now but these capacities on each ferry have been arrived at through our expert marine managers, working with health and safety managers and the Scottish Government.
"This has been an exhaustive and scientific process designed to put passenger and staff health at the top of our priorities.”
“At no stage has the MV Isle of Mull ever run to its capacity during these restrictions and the changes from 2m to 1m announced last week have increased the capacity of the vessel to 35 per cent of its normal numbers.
"The outer deck and lounge areas are open under distancing rules and the only area closed is the forward cafe which has had to be allocated to allow staff to take their meal breaks in line with distancing measures.”
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