The new dates announced mean the workers, who have just passed 50 days of action, will continue until at least 1 June.
Unison says the long-running dispute centres on the trust's refusal to properly re-evaluate the roles phlebotomists perform, which the union believes should be paid at a higher grade. This amounts to a breach of the trust's own policy and the NHS's job evaluation scheme, the unions says.
Talks took place on 13 May to resolve the dispute, but the trust made clear it would only allow a job evaluation panel to look into a new, hypothetical job description for the future, not the tasks phlebotomists have been doing for years.
Staff are not asking for a new role, but want their current jobs fairly paid using the NHS's own process, says the union. At present they are paid at band 2 of the NHS's Agenda for Change pay scale but should be on the higher band 3, Unison says.
The co-chair of the NHS staff council has given written advice that the trust is breaching national agreements by blocking the panel, says the union.
The phlebotomists have now extended their strike action to 1 June, but remain available for talks if the trust is ready to follow the correct process, the union adds.
Unison South West regional organiser Chris Roche said: "Phlebotomists at Gloucestershire hospitals have the skills, knowledge and experience that clearly meet the criteria for a band 3 role.
"They carry out complex procedures, care for patients with additional needs and work independently across wards and clinics.
"They have done this vital job for years and they deserve to be recognised and paid accordingly."
In response, a Gloucestershire Hospitals NHS Foundation Trust spokesperson, said: 'We welcomed the opportunity to sit down with Unison and our phlebotomy colleagues to try and resolve the current issues.'
They added: 'The trust position is that the phlebotomy job description remains aligned at a Band 2, which is consistent with other trusts in Gloucestershire, South West and across the country, who continue to use the Phlebotomist Band 2 National Profile.
'We look forward to further discussions with Unison and our teams.'
