The studies so far have not been able to rule out completely the possibility that the patients benefited from the placebo effect of receiving close medical attention.

However, Prof Muir told the BBC: "It seems odd that it should all just be chance and a placebo effect. We are seeing things that are interesting and somewhat surprising.

He added: "My expectation had been that we would see very little change and if we did see change it would be a relatively short-lived temporary change. [But] we have seen changes that have been maintained over time.

The patients in the study, which took place at Glasgow's Southern General Hospital, were aged in their 60s, 70s and 80s, and all had suffered their stroke between six months and five years earlier.

Frank Marsh, 80, a retired teacher, was once a keen piano player, but a stroke five years ago left him with poor strength and co-ordination in his left hand.

Describing the improvement he experienced after receiving the stem cell treatment, he told the BBC: "I can grip certain things that I never gripped before, like the hand rail at the baths, with my left hand as well as my right.

"It still feels fairly weak and it's still a wee bit difficult to co-ordinate but it's much better than it was."

He added: "I'd like to get back to my piano. I'd like to walk a bit steadier and further."

The small-scale study, whose results will be presented to the European Stroke Conference in London tomorrow, is the worlds first clinical trial examining the safety of neural stem cell treatment in stroke patients.

The first phase of the research was carried out to check that the procedure is safe. Now Prof Muir and his team are planning a larger phase two trial later this year to explore further how effective the treatment is.

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Stroke patients show signs of recovery in stem cell treatment trial