The action by Unite the Union members, which follows an Employment Tribunal outcome for a former individual colleague, began on 15 June and is being extended until 4 September.

Unite regional officer Helen Flanagan said: ‘Workers at these hospitals are at the end of their tether and have lost all confidence in management to resolve this issue. They have been left with no choice but to take strike action to get their legally entitled holiday pay.'

A spokesperson from the Lancashire and South Cumbria Pathology Service, which provides blood sciences service for Lancashire Teaching Hospitals NHS Foundation Trust and East Lancashire Hospitals NHS Trust, said the dispute focused on a national issue that could not be resolved currently although talks remain ongoing.

The spokesperson added: ‘We have always been clear that we respect the right of all colleagues to take industrial action. Our focus is on maintaining safe and high-quality services for patients. This is particularly important in blood sciences, where timely access to laboratory services is essential for providing blood transfusions, haematology services, biochemistry services and blood testing capacity.

‘Colleagues who remain in the labs are working extremely hard to keep services running and we would like to thank them for their ongoing support at what is a really difficult time.

‘Our priority is to restore blood testing capacity and the full range of tests to pre-industrial action levels as quickly and safely as possible. Capacity is being increased in a phased way, with appropriate testing and assurance processes in place to ensure patient safety, working closely with partner organisations.'