This result has important implications for the care of patients worldwide.

Prof Rory Collins Oxford University

Both are potentially dangerous conditions that are more likely to happen after major operations.

And as the condition often arises once the patient is back home after surgery, it can be particularly dangerous.

The Pulmonary Embolism Prevention (PEP) Trial - involving more than 17,000 patients, found that there were 43% fewer cases of pulmonary embolism in those taking aspirin, and 29% fewer cases of deep-vein thrombosis.

The study was reported in The Lancet medical journal.

Aspirin prevented a calculated four deaths for every 1,000 people given the drug, and the researchers suggested that it be routinely given to patients before major operations.

Aspirin was thought responsible for some cases in which there was excessive bleeding.

However, the other drug used by surgeons to reduce the chance of clotting - heparin - appears more likely to cause this sort of bleeding.

Important results

Professor Rory Collins, from Oxford University, said that the results of the trial were important.

"Pulmonary embolism is a life-threatening complication after surgery, and the PEP trial shows that it can be prevented by aspirin.

"This result has important implications for the care of patients worldwide.

"Aspirin has a number of advantages. It is inexpensive and can easily be taken for prolonged periods - this is important as pulmonary embolism after surgery often occurs following discharge from hospital."

Professor Colin Prentice, from the University of Leeds, said: "Aspirin can cause bleeding, but the PEP results are likely to extend to patient having other types of surgery and to people at risk of pulmonary embolism for other reasons."

Aspirin, a 100-year-old drug, and one of the cheapest in the NHS, has been shown to have a number of beneficial effects.

At low doses, it interferes with the ability of the blood's platelets to produce a clot.

However, higher doses, particularly over a sustained period, can lead to gastrointestinal problems, or even strokes caused by bleeding into the brain.