The 37 phlebotomists are to walk out for nine more days after Gloucestershire Hospitals NHS Foundation Trust failed to resolve a long-running dispute over their pay and grading.
The next round of action will take place next week on 24-27 March, followed by five days the following week (31 March to 4 April), this comes after three days of strike action this week.
The union says the trust has ignored a fair job evaluation process and attempted to get out of its obligations to assess the phlebotomists' skills and responsibilities.
Unison South West regional organiser Chris Roche said: "Phlebotomists play a crucial role in patient care, but the trust is treating them with contempt. Instead of following the proper process to evaluate their jobs properly, hospital managers are trying to dodge the issue.
"These NHS workers have already taken strike action, yet the trust continues to delay. Its managers have had more than a year to put things right, but have chosen to ignore the evidence and the workers themselves. Now, faced with yet another failure by their employer, phlebotomists have no choice but to escalate their action."
In response, trust chief executive Kevin McNamara, said: ‘We recognise and appreciate the valuable contribution that our phlebotomists make in providing patient care and we will continue to work closely in partnership with our local union representatives and phlebotomists on resolving these issues.
‘Contingency plans are in place and we are advising patients who have an appointment at either Cheltenham General Hospital or Gloucestershire Royal Hospital during periods of industrial action to attend as normal unless they hear from us directly to advise otherwise. However, there will be no phlebotomy service / blood taken from patients during industrial action.'