The Rail, Maritime and Transport union (RMT), the biggest at the firm, accused its Scottish Government owners of scuppering progress towards a deal.
It said a meeting with managing director Robbie Drummond and other unions today following two rounds of pay talks had ended in failure.
The RMT said the pay offer amounted to a pay cut which it described as “derisory” and a “serious kick in the teeth”.
The Transport Salaried Staffs Association (TSSA) this week also rejected the 0.5 per cent offer, along with fellow CalMac unions Unite and Nautilus.
An alternative, two-year deal, involving a RPI inflation rate rise in the second year, was also turned down.
RMT general secretary Mick Cash said: “Our negotiators have tried over a series of meetings to get a cost of living award for essential lifeline ferry workers.
“Despite their best efforts again today, the CalMac managing director has given the same response as his management colleagues have issued from the first meeting on October 7.
"It’s become increasingly apparent that Transport Scotland and the Scottish Government are putting pressure on CalMac to hold a line which will see our members financially disadvantaged.
"This is not acceptable to RMT and we will be balloting our members to secure financial fairness for members and their families in these uncertain times.”
TSSA general secretary Manuel Cortes said “CalMac are behaving like Scrooge, offering what amounts to a pay cut just as people are thinking about Christmas.
“Last week, Robbie Drummond, sent a glossy postcard to every member of staff, thanking them for their ‘support, understanding and commitment’ serving the island communities throughout the pandemic.
"Well, talk is cheap and this week we’ve found out exactly how cheap Robbie Drummond’s talk really is.
“Clearly, 0.5 per cent is a paltry offer to key workers who’ve kept the ferries running, bringing vital lifeline supplies to the island communities, in the middle of a global pandemic.
"Our members are absolutely right to reject that offer.”
CalMac managing director Robbie Drummond said: “CalMac has suffered significant and unprecedented financial pressures due to Covid-19 and it is likely that these will continue to have an impact for some time.
“Despite this, we are committed to rewarding staff in recognition of their determination to provide a high quality and safe lifeline service during an extremely difficult period.
“We are thankful for the support from Transport Scotland under our contract, which has enabled us to protect every job during the pandemic and to provide furloughed employees 100 per cent of their pay.
"We also took steps to ensure their safety and well being was well protected.
“This pay offer is the same as the current rate of inflation and is being made with our staff in mind – providing a fair and generous offer in the current circumstances.
"This offer ensures the best possible route for us all as we get through the pandemic together.”
CalMac won an eight-year contract in 2016 worth a total of some £900 million, in a contest against private sector outsourcing giant Serco.
A Transport Scotland spokesperson said: “CalMac pay policy sits outside the coverage of the Scottish Government’s pay policy and, as such, this is a matter for CalMac and the unions concerned.
"We would urge management and unions to continue to engage in meaningful dialogue to reach a mutually agreeable outcome that avoids impacting staff and ferry users at an already difficult time for communities served by CalMac.”