Key staff at Raigmore Hospital will decide next week whether to go on strike over changes to their shifts.
Porters at the biggest hospital in the north will be balloted for industrial action within days.
The GMB union confirmed the move yesterday, saying that NHS Highland had so far refused to meet its officials and dispute mediators Acas.
The Press and Journal reported on Saturday that a consultative ballot held by the union had found that 97% of the porters at Raigmore were in favour of taking action.
The new ballot will ask the porters to back strike action and alternative industrial action short of a strike.
The dispute centres on a new rota, due to be trialled by NHS Highland from August 1, which involves porters working to a 16-week pattern of shifts, with seven different start times.
They currently work an 11-week rota, with four start times.
One of the main issues raised by staff is that the new system will involve porters working seven days on for eight weeks.
Staff say that the longer spells are likely to result in more sick leave because of the physically demanding nature of the job, leaving a thinly-stretched service at greater risk.
Liz Gordon, regional officer for the GMB, said NHS Highland had so far refused a meeting with Acas, or its request to postpone the trial.
“We will be giving them notice of our intention to hold a ballot next week,” she said.
“We’re still holding out for Acas. We’re still hoping we can sit down and talk it out. The NHS so far are not willing to go to Acas.”
Porters act as a “bridge” between hospital departments, performing a number of tasks, including moving medical equipment, removing the bodies of deceased patients from wards, dealing with waste, as well as delivering meals.
Ms Gordon said: “It’s with enormous trepidation that they are doing it. It’s not something they want to do at all.
“It’s unheard of in this part of the world for a group of staff to take action outwith national action. This is very unusual. I’ve never encountered it in 20 years of working here.”
An NHS Highland spokesman said: “Senior management from NHS Highland will meet with GMB to discuss the situation in due course.”