Radiographers at St Helens and Whiston hospitals will strike this morning as part of an ongoing dispute.
NHS bosses says some appointments may be affected by the walk out, which will for four hours on Monday.
A hospital spokeswoman said: “Following the results of a trade union ballot by the Society of Radiographers, a four hour period of industrial action will take place on Monday, October 20, from 9am until 1pm.
“The Trust is working closely with all staff and other local NHS organisations to minimise any disruption to hospital services.
“All urgent and emergency care will continue to be provided at St Helens and Whiston hospitals and at the Walk-in Centre, St Helens town centre.
“However, we ask patients to consider the seriousness of their condition prior to attending the Accident and Emergency Department, which may impact on radiology services.
“Whilst some non-urgent radiology appointments have been re-scheduled, those patients affected have been contacted and will be seen as soon as possible.
“Patients who have been advised by their GP to come to the hospitals for a routine x-ray are asked to attend after 1pm on this day.
“If you require any further information please contact the Trust’s Radiology Department in: 0151 430 1866 or visit the Trust website: www.sthk.nhs.uk.
“We apologise for any inconvenience caused by this national strike action.”