The postal strike has left blood stocks in Ulster at dangerously low levels, it was warned last night.
The Northern Ireland Blood Transfusion Service warned that the levels of group A negative in particular were close to running out because they have been unable to send out invitations.
Spokesman Paul McElkerney said the ongoing Royal Mail dispute is making it difficult to contact donors with specific blood types.
"Over the past week, stocks of blood group A negative fell to less than two days' supply and whilst the Service has a number of mechanisms for dealing with demands from hospitals, a key action is to send reminders by post to donors," he said.
"This has not been possible and we are also concerned that any backlog of mail may cause delays for routine donor session invitation letters."
The service is calling on donors, especially blood group A negative, to attend a local blood donation session. Details of sessions can be found on UTV Teletext page 602, by visiting nibts.org or by calling the service on 0500 534 666.
Meanwhile, Ulster postal workers have pledged they will take to the streets in protest if talks to resolve an industrial dispute drag on.
Negotiations with the Royal Mail resumed yesterday as up to 130,000 employees across the UK embarked on a second 48-hour strike.
Strikers mounted pickets at mail centres across Northern Ireland and promised they would take their message onto the streets if no compromise is met.