Unite said protests will be staged during the strike days on 3-5 September outside Downing Street, The Department of Health and Social Care, NHS England and London Bridge Hospital.

Unite general secretary Sharon Graham said: ‘Guys' and St Thomas' theatre nurses know crushing workloads are an issue across the whole of the NHS. The changes to shift times at the trust mean they are at the sharp end of reckless attempts to reduce waiting lists, with no apparent concern for the health and wellbeing of the staff providing the care.

‘Working devoted staff into the ground will make things worse, as dedicated NHS workers leave for the sake of their own health. That is the nurses' message to Guys' and St Thomas', which must reintroduce sustainable hours, and to the Government, which cannot expect to fix the NHS by shovelling ever increasing demands onto already exhausted staff.'

The union said an hour-long extension 8-9pm earlier this year had followed the extension of 7-8pm and the introduction of Saturday working.

A spokesperson for Guy's and St Thomas' NHS Foundation Trust said: ‘The shift times have been changed to finish at 9pm on some days, however, staff are not being asked to work any more hours.'

The trust said the small number of staff who had been rostered for ‘standby shifts' would receive an additional flat payment regardless of whether they worked or not and would receive additional pay for working beyond 8pm.

The trust said rostered standby shifts also offered the predictability of when a nurse might work later than 8pm so they can make appropriate arrangements.