Patients having routine hip or knee replacements are being urged to ask for their own blood to be recycled after surgery.

Many people are unaware the option exists at several NHS hospitals, although it is safer than transfusing a donor's blood, according to the Patients' Association, an NHS rights charity.

The group is encouraging people to choose the technique, known as post-operative cell salvage, saying it would help to eke out supplies in blood banks, especially since controls to stop the human form of BSE spreading through transfusions mean there are fewer donors.

The technique filters debris from salvaged blood before transfusing it back to the patient on the ward. In orthopaedic surgery, much of the blood loss happens soon after the operation, and the blood is relatively clean. However, it is not suitable where operation sites are infected. Blood can only be retransfused up to eight hours after wounds are closed, and there is still a need to train staff in its use.

It is one form of autologous transfusion - where a patient gets his or her own blood - being considered following government orders to hospitals nearly three years ago.

The association has a website, (yourblood-yourchoice.com), with backing from Summit Medical, a transfusion product manufacturer, to promote the choice of post-operative recycled blood.

Simon Williams, the association's director of policy, said: "Not many people know beforehand about the blood transfusion choices they have, so we are keen to draw attention to the benefits and help them find out more, so that they can make informed choices with their doctors."

Studies suggest patients leave hospital sooner and feel more reassured, and the risk of a potentially fatal mistake when a patient gets the wrong type of blood is averted, the association says. "Medical evidence clearly shows there are many advantages to getting your own blood back where it is possible" said Mr Williams.