The Red Cross Blood Service is appealing for donors of O-negative blood to step up and give, as supplies run low.

A critical shortage of O-negative blood across Australia has sparked the latest Red Cross Blood Service appeal for donors to come forward.

The blood service is urging people with O-negative blood to make a donation as its reserves dip to just two days' supply.

An increase in demand for O-negative blood, combined with a fall in donor numbers due to cold and flu symptoms, is behind the decline in supply, with as many as 1000 cancelled donations per week.

Spokesman Shaun Inguanzo said the blood service was calling on 4500 additional O-negative donors to come forward and give blood between now and the end of July.

"O-negative is a universal blood type that can be used in an emergency situation where the patient's blood type is unknown," Mr Inguanzo said in a statement on Tuesday.

"The number of people suffering cold and flu symptoms greatly impacts the number of regular donors who are able to give.

"We need others to take the place of those who will be unable to answer our call."