The researchers claim the experimental drug, called Ancrod, lowers levels of a blood-clotting substance in the blood and may be able to reverse the effects of a stroke.

It could also protect against further strokes and is less likely to cause internal bleeding than existing clot-busting drugs, they claim.

In a study of 500 stroke patients it helped 42% recover their physical and mental abilities within three hours.

Of those given an inert dummy drug, 34% regained their previous faculties.

Pit vipers

Ancrod is derived from the venom of a family of snakes known as the pit vipers.

These include deadly rattlesnakes, such as the Diamondback, that live in the US and Mexico.

Researchers discovered that the blood of people bitten by rattlesnakes failed to clot.

Based on that observation, the venom was extracted and turned into an anti-coagulant.

Ancrod is not yet approved by the American Food and Drug Administration for stroke treatment.

The only FDA-approved acute stroke treatment is Tissue Plasminogen Activator (TPA).

TPA is administered in a single-dose, hour-long injection, whereas Ancrod has to be given intravenously through a catheter over three to five days in hospital.

While TPA dissolves clots, Ancrod lowers the levels of the clotting factor fibrinogen in the bloodstream.

Professor David Sherman, from the University of Texas, San Antonio, said: "We found that patients who had their fibrinogen levels lowered promptly, within six hours, and maintained in the target range after treatment, had the best response to the treatment.

"What I envision if Ancrod is approved is that the physician would have the option of using either TPA or Ancrod based on what would be best for that particular patient.

"Overall, TPA has been shown to be a little bit more effective than Ancrod, but it also carries a little bit more danger with the possibility of bleeding in the brain."

Cautious welcome

The Stroke Association issued a statement in which it welcomed any medication that could potentially reduce death and disability from stroke.

"However, it is important to realise that the snake venom preparation needs to be administered very quickly after a stroke," the statement said.

"Unfortunately, in the UK, the systems are not in place to allow this to be done easily."

The association said some blood clot 'busting' drugs were already on trial, but these needed to be administered at hospital within three hours of a stroke taking place.

Stroke is the third most common cause of death in England and Wales after heart disease and cancer.

Stroke is also the largest single cause of severe disability in England and Wales, with over 300,000 people being affected at any one time.

Rattlesnake venom has been found to be effective in other drugs, including Integrilin, a heart drug.