It is restricting blood orders to hospitals to the minimum possible and warns that the next move could be to cancel all operations except emergencies.
Donations in London have dropped by between 10 and 20 per cent which has coincided with a run on blood following the recent cold snap.
Christmas and New Year is traditionally one of the most difficult periods of the year for the blood transfusion service, where stocks can fall by as much as 30 per cent. But Peter Gibson, head of communications of the London and South- east zone said that it was "unusual" that they were experiencing problems two to three weeks before the Christmas period.
"This is very low going into the Christmas period," said Mr Gibson. "All it needs is a serious incident and we will be under a great deal of pressure and we cannot rely on other parts of the country to help us out."
The London and South-east service launches its Christmas appeal today. Every day 10,000 units of blood a day are needed to supply hospitals but at present only 6 per cent of the population donate blood.
Nationally, figures are slightly down with the National Blood Authority holding 13,500 units yesterday instead of 15,000 but a spokesman said there was no question of a national crisis.
In Scotland blood supplies are also running desperately low as hospitals battle to cope with the E-coli 0157 outbreak, the Scottish National Blood Tranfusion Service(SNBTS) said yesterday. At the launch of their national Christmas blood appeal, SNBTS said the outbreak was placing one of the "biggest ever" demands on blood products. "I think it is one of the biggest ever demands for blood products we have had. We just have to repeat our urgent message to encourage people to come forward and give blood," it said.
The heaviest demand has been for plasma products where toxins produced by the E-coli bacteria are washed out the system using healthy plasma from frozen supplies.
n New donors are advised to get in touch with their local blood service if they wish to donate, or ring the National Blood Authority on 0345 711 711.Reuse content