The 37 workers voted for strike action after Gloucestershire Hospitals NHS Foundation Trust failed to resolve a dispute over staff pay and banding that began more than a year ago.
Unison says the trust has incorrectly calculated the NHS workers' pay, leaving them on the lowest salary band of the NHS, despite their skills, knowledge and daily tasks warranting the next band up.
The phlebotomists believe they are paid around £1 per hour less than they should be, according to Unison.
The union says hospital managers promised to address the issue by the end of February, but, despite talks, the employer has failed to resolve the matter.
Unison South West regional organiser, Chris Roche, said: ‘The trust has had over 12 months to correct the pay of their staff, but has instead chosen to miss deadlines and break promises.
‘Phlebotomists are vital to the health service and deserve much better than the treatment they've received. They should be paid fairly for the work they do, not left struggling to pay their bills.
‘NHS staff want to be at work caring for their patients. They don't want to be on strike and have tried everything to get hospital bosses to do the right thing. But the phlebotomists now feel they've no choice but to stand up for themselves and the health service they provide. Hospital managers must act quickly and pay the wages their staff deserve.'