The technique will allow patients whose brains have degenerated to grow replacement cells for the damaged tissue.

Patients with Parkinson's lack cells that release the chemical dopamine. Dopamine carries messages around the brain.

The discovery could lead to doctors being able to offer unlimited supplies of dopamine-secreting neurons.

Cloned cells

The work involves stem cells, which are basic cells with no particular function. They can be cloned and grown in the laboratory.

Doctors have been working on them because they hope the cells can be coaxed to take on the function of more specialised cells.

The discovery, which was made in experiments using rats, comes less than a week after the government reject calls to let scientists clone tissue from embryos for such experiments.

The decision was criticised by some scientists, who said it would leave the UK behind in a field that is likely to have potentially huge medical benefits and commercial rewards.

Advanced start

Dr Ernest Arenas, from the Karolinska Institute in Stockholm, Sweden, led the research and published his findings in the journal Nature Biotechnology.

The stem cells he used were more advanced than those taken from embryos - they had already started on the path to becoming neurons.

The scientists aimed to persuade them to become the right kind of neuron for their purposes - dopamine secreters.

Replacement tissue derived from the patient's own stem cells would have the advantage that they would be unlikely to be rejected after being implanted.

Two stage process

The scientists identified two stages to making the cells take on the desired characteristics successfully.

First they had to be genetically modified to make them susceptible to the effect of other triggers.

Then they were grown in the presence of other specialised brain cells which caused them to grow into dopamine secreting neurons.

The scientists said: "The procedure we describe . . . might be used to engineer neurons with the desired neuro-chemical (form) as a source of material for neuronal transplantation in the treatment of neurodegenerative diseases.