Tens of thousands of medical procedures and appointments are being cancelled this week due to strike action by medical scientists.

The Health Service Executive is warning of significant disruption and service delays in hospitals across the country due to the industrial action.

Medical scientists will go on strike tomorrow and on Wednesday as they step up their campaign over long-standing pay and career development issues.

It follows a one-day work stoppage last week.

The union that represents medical scientists, the Medical Laboratory Scientists Association (MLSA), says no approach has been made by the HSE or the Department of Health since last week's strike.

This HSE says this week's action will lead to the cancellation of many inpatient and day-case elective procedures as well hospital outpatient appointments across the country.

All routine GP testing services will be suspended on both days and patients currently in hospital requiring tests will be affected.

Delays in Emergency Departments can also be expected.

The HSE says dialysis and some cancer services will continue.

Affected patients are being contacted directly and people are being asked not to phone hospitals but instead check the HSE website for updates.

"While efforts are continuing to try to avert this action, the HSE is working with the MLSA to ensure arrangements are in place on the days for the provision of a limited range of services safely," the HSE said in a statement.

Medical scientists carry out critical diagnostic testing of patient samples with the majority of them working at laboratories in public hospitals.

The action on Tuesday and Wednesday will again involve the withdrawal of routine laboratory services from 8am to 8pm on both days.

The MLSA says the vast majority of its 2,100 members will be on picket lines this week, at all public voluntary and HSE hospitals.

For the first time in this dispute, pickets will also be placed at the Irish Blood Transfusion Service.

The IBTS says non-urgent patient testing services will be suspended during the days of industrial action.

The service says contingency plans are being put in place for tomorrow and Wednesday that will see collection teams continue to run blood donation clinics as scheduled.

Testing and processing of all donor blood will continue as normal, and all blood and blood component orders from hospitals will be processed.

"These contingency measures are designed to maintain the sustainability of the blood supply during the dispute. Any donor with an appointment to give blood this week should continue to do so," the IBTS said in a statement.

If no progress is made, the MLSA says a further three days of strikes are planned for next week, on 31 May, 1 June and 2 June.

Medical scientists say they are paid less than colleagues who carry out the same work and, in some cases, less than staff in lower grades.

They also claim they have fewer career progression opportunities and that these issues are leading to a recruitment and retention crisis in their sector.

"Since last week's action neither the HSE nor the Department of Health have come to us with a meaningful proposal or invitation to talks that could address the issues involved," said MLSA General Secretary Terry Casey

"It is because of this that members throughout the country are stepping up the action," he added.