Scotland's blood banks have less than a three-day supply of a universal blood group which can be given to anyone in an emergency.
The Scottish National Blood Transfusion Service (SNBTS) said its supply of the O Negative type is below half the ideal six-day stock it aims to have of all eight blood groups.
O Negative can be given to anyone in an emergency, and is the only safe option when a patient's blood group is not immediately available to medical staff.
It can also be given during specialised procedures for unborn babies.
The service is urging donors of all groups, particularly O, A and B, to give blood ahead of the October school holidays, to allow it to build supplies.
Dr Moira Carter, from SNBTS, said: "Although we are currently able to meet hospital demand we need to give our stock levels a significant boost before we enter the holiday period.
"This year we are particularly concerned as we expect that more families will head off for some much-needed sunshine given the poor summer.
"When that happens we find that donor numbers fall by up to 20%."
SNBTS director Mary Morgan said: "Blood groups O and A are the two most common, therefore they are the most needed by Scotland's patients.
"O Negative donors are uniquely placed to help patients when they need it most as their blood group can be given to anyone in an emergency."
For more information on donating, visit www.scotblood.co.uk.
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