The National Blood Service aims to keep 20,000 units of blood in reserve - equivalent to two days' donations - but this has fallen to 15,000 units. Unless 2,000 more donors a day come forward, some hospitals may be forced to cancel operations.
All regular donors are being urged to keep their appointments and the NBS is appealing for new donors.
A spokeswoman for the service said supplies were being hit because hospitals had stepped up the number of operations and procedures needing blood over the past year.
'We are pretty close to an all-time low . . . We have boosted our collections but we are still struggling,' she said.
The NBS is collecting blood from 8,000-9,000 people a day but needs to achieve a daily target of 10,000 donations to top up reserves and meet the current demand.