A revolutionary device which provides sufferers of Parkinson's disease with an on-off switch to control their symptoms has been launched in Britain.

The machine consists of a battery-powered transmitter connected to electrodes embedded deep in the brain. By turning the device on with a matchbox-sized magnet, the disabling symptoms of the illness - involuntary shaking, lack of balance, and immobility of limbs - can be controlled.

Alternatively, patients can reduce the power setting so that their symptoms are reduced.

Up to 20,000 people a year in the UK could, in theory, be helped by the treatment but few are likely to be offered it because of cost.

The Activa device, made by the American based company Medtronic Inc, costs around #5000 and the surgery required to instal it another #7000 to #10,000.

Maintenance, including renewing the battery, might cost several more thousands of pounds every five years or so.

The Parkinson's Disease Society and doctors pioneering the system claim that, balanced against the high cost of medication and long term care, it could be worth the price. Drugs for controlling the symptoms of the disease can cost #15,000 a year.

About 125,000 people are diagnosed each year in the UK.