Professor Keith Muir, from Glasgow University, said the results were "not what we would have expected" from the group of patients, who had previously shown no indications of their conditions improving.

He stressed it is too soon to tell whether the effect is due to the treatment they are receiving.

The trial involves injecting stem cells directly into the damaged parts of the patients' brains, with the hope they would turn into healthy tissue or kick-start the body's own repair processes.

Nine patients, in their 60s, 70s and 80s are taking part in the trial at Glasgow's Southern General Hospital to assess the safety of the procedure.

Among the patients to have shown improvements is former teacher Frank Marsh, who had a stroke five years ago.

The stroke left him with poor strength and co-ordination in his left hand, and poor balance. He needs a walking stick to help him move around the house.

The 80-year-old took part in the trial at Glasgow's Southern General Hospital, and said he had seen improvements in the use of his left hand.

Mr Marsh said: "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."

Mr Marsh said he hoped the improvements would continue, adding: "I'd like to get back to my piano. I'd like to walk a bit steadier and further."

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Stem-cell trial gives fresh hope to stroke patients