Biomedical scientists who analyse blood tests for coronavirus patients are taking industrial action after claiming they are being underpaid.
Some pathology staff at Royal Preston and Chorley and South Ribble hospitals in Lancashire will "work to rule" for three months, which could delay some samples, the Unite union said.
The row is about staff being asked to work unsupervised without better pay.
Lancashire Teaching Hospitals NHS Trust said it was "mystified" by the action.
It said it had upheld the union's original grievance and offered to backdate staff's pay but the union had introduced a new complaint about staff completing specialist training before progressing in a pay band.
Unite told the Local Democracy Reporter Service the staff should be moved up a pay band when they start to operate without supervision, a move which would see their annual wage increase from £30,000 to just under £38,000.
North West regional officer Keith Hutson said members had been left in the lower pay bracket long after being expected to work alone at night and weekends.
He said affected staff "should be moved over to the appropriate level, not put on the bottom of it".
He added: "The inability for people to have done [specialist training] is the fault of the employer, so until they address that part of the grievance, it is not over."
The trust said it was "extremely disappointing" that staff had voted for industrial action "in the midst of a pandemic, particularly when so many other staff are going above and beyond the call of duty every single day."
"We think most people will be totally mystified as to why the union is advising staff to adopt this approach."
Unite said members stopped short of a walk out so that priority tests were not held up.
They would "strictly adhere" to the job description for 12 weeks from 7 December, it said.