The staff say they regularly work shifts without proper breaks or time off, according to Unite. On occasion, the workers are not getting the legal minimum 11-hour rest period, between shifts, the union claims.

Unite general secretary Sharon Graham said: ‘Like much of the NHS this department is under resourced and understaffed, and this is now seriously affecting the wellbeing and safety of the staff and patients. This is completely unacceptable and dangerous.

‘Our Unite members have the full support of their union in this dispute.'

In response, a spokesperson for NHS University Hospitals of Liverpool Group, said: ‘Patient safety is our main priority, and we appreciate the hard work of colleagues to ensure the timely processing of samples at our hospitals.

 ‘As an organisation, we respect colleagues' right to ballot for industrial action and we are liaising closely with Unite to listen to and understand staff concerns.'

The Unite ballot closes on 15 April and strikes could begin in late April.