An amber alert has come into force for the first time as stocks of blood in Scotland have dropped to unprecedented low levels due to the shortage of donors.

The warning has been sent to hospitals across the country by the Scottish National Blood Transfusion Service (SNBTS), urging medical staff to only use Type O blood when essential.

Type O blood is particularly important for transfusions as O-negative acts as a universal donor and O-positive is the most common blood type in the UK, with about 35% of the population having it.

Stocks have consistently been below target levels for several weeks and Group O blood has fallen to below two days of reserves on several occasions recently.

SNBTS aims to always retain 5 – 7 days supply of all blood groups.

The alert comes after a decline in people joining the blood donation register.

In the past decade SNBTS said there had been a 24% drop in blood donors - about 30,000 people.

The blood transfusion service said it was particularly struggling to attract younger donors.

Hospitals are estimated to need about 135,000 units of blood each year to treat patients.

The SNBTS said that to do this appointments at its five national donor centres in Aberdeen, Dundee, Edinburgh, Glasgow and Inverness needed to be fully booked all year round.

However, an average of just 85% of these appointment slots are filled.

Debbie McNaughton, director of donor and transport services at the SNBTS, told Radio Scotland Breakfast that currently, about 2% of Scotland's population are signed up to give blood, with the majority donating once a year.

"If they could be donating maybe twice or three times a year, that would hugely help to provide resilience for the blood supply," she said.

McNaughton went on to urge anyone who is healthy and 17 or over to sign up to the donor register.

The amber alert will trigger new protocols in hospitals to minimise non-essential usage of O-type blood, but emergency operations, blood transfusions, transplants and cancer surgeries will continue as normal.

The alert has initially been issued for an initial period of 4-6 weeks but will be kept under close review.