The partial strike by biomedical scientists is part of a long-running pay dispute.
Their union Unite claims the trust has backtracked on a 2019 agreement to upgrade the 21 staff to a higher grade on their specialist pay scale.
They analyse patient blood samples at the Royal Blackburn and the Burnley General Teaching Hospitals.
After an 85 per cent vote for industrial action the scientists will stop doing night, weekend and late shifts for a month.
They will only work on their core days – Monday to Friday from 8.45am to 5pm and early shifts from 7am to 3pm from May 7 until June 4.
Kate Quinn from the trust said any industrial action would not affect emergency care at the Royal Blackburn and said many staff had been regraded.