The sensors detect tiny changes in metabolism and transmit data, via a mobile phone, to the patient’s doctor.

Scientists at Imperial College London who invented the device believe it will enable some patients to lead a normal life while being kept under constant watch.

It has the potential to be developed into a complete body sensor that could be implanted into normally healthy people to pick up early signs of disease.

The sensor, which includes a Pentium microprocessor just 2mm square, will initially be implanted in diabetics. Trials will begin by Christmas at St Mary’s hospital, London. The implant will be programmed to send an emergency text message via a mobile phone, alerting medical staff to changes in blood-sugar levels.

If the problem is serious, the patient will be given immediate medical advice. Once patients become familiar with the system, they could monitor their condition themselves.

The only restriction is that the computer’s low power output means that it needs a receiver — generally a mobile phone — to be within a metre of the patient to pick up the sensor’s wireless signal from its miniaturised antenna.

Chris Toumazou, director of the Institute of Biomedical Engineering at Imperial, is hoping eventually to link the sensor to an insulin pump that can be operated remotely by a doctor. The sensor could also be used to protect people with heart and respiratory diseases. The researchers are exploring ways to detect chemical changes in a patient’s blood.

“The computer in your body can take away anxiety and allow medics to take control of your care from miles away,” said Toumazou.

More than 17.5m people in Britain have one or more chronic diseases of varying severity — a figure that is set to soar as the elderly population grows over the coming decades. If many of these patients could be turned into experts monitoring their own conditions with minimal intervention by doctors or nurses, it could free up significant NHS resources.

The aim is also to develop the system so that the sensor can provide prompts to patients to take medication.

Pathology departments are under particular pressure because of the increase in the number of chronically ill patients who need regular blood tests.

Oracle, the technology company that is backing the project, has designed the software to be compatible with the NHS’s new £6.5 billion computer system.

This will allow the data to be stored on a patient’s record and accessed by healthcare staff nationwide.

Jeremy Nettle, the European healthcare director of Oracle, said: “These devices are going to give patients who have to make regular visits to GPs and nurses much greater control and independence.”

He added: “Our aim is eventually to get the cost of each sensor down to £1 so that the technology is available to everybody.”

The Imperial team has developed four other prototypes that rest on a patient’s skin. They include sensors to detect heart disease, high blood pressure and hypothermia and motion sensors, used to monitor housebound old people.

Scientists in America implanted microchips containing medical records under the skin of human guinea pigs last year. The records could then be read by a doctor using a scanner.

Another American company has placed a microchip on a pill bottle, which plays back spoken prescription advice through a speaker.

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