Transplanting cells from aborted foetuses was shown to reverse the effects of the progressive neurological condition in studies in the 1990s but trials were abandoned because the results were unpredictable.

While some patients found some relief from tremors, rigidity and slowness of movement following surgery, others developed jerky, involuntary movements known as dyskinesias.

The side effect was so serious that the human trials were halted in 2001.

Researchers funded by the Medical Research Council (MRC) and Imperial College London now believe dyskinesia was caused by malfunctioning serotonin cells and say the disorder can be treated with drugs.

Dr Marios Politis from MRC Clinical Sciences Centre at Imperial College London, who led the research team, said: "After the huge excitement surrounding the potential of brain cell transplants in the 1990s, we are thrilled that this discovery could reopen the door to this promising area of research.

"We know that the benefits of this treatment could last up to 16 years, and we look forward to bringing this treatment one step closer to a reality for Parkinson's patients."

Dr Politis said earlier trials showed the surgery could produce "remarkable improvement" in some patients but the majority suffered serious disabling adverse effects.

He said dyskinesia is a common side effect of drugs used to treat the symptoms of Parkinson's disease but the patients were suffering involuntary uncontrolled movements while off their drugs.

The new research, which is published online today in Science Translational Medicine, scanned the brains of two Parkinson's sufferers who suffered dyskinesia following surgery.

Dr Politis said researchers found the side effect was the result of an overabundance of serotonin cells in the transplanted tissue.

This could be prevented through drugs, he said, adding that the team suggested serotonin cells be removed during the preparation of transplanted tissue in future trials which they hope will now be able to take place.

Dr Politis said the findings represent "a major advance in the field of cell therapy for Parkinson's disease".

The transplant is not currently available as an option for treatment in the UK .

There is currently no cure for Parkinson's, which affects around 120,000 people in the UK , according to the charity Parkinson's UK .

People with Parkinson's do not have enough of the chemical dopamine because some nerve cells in their brain have died but it is not yet known why that happens.

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