A satellite tracking scheme designed to monitor the movements of violent offenders with mental health problems is being expanded, following a successful pilot.
The system is intended to prevent psychiatric patients absconding or offending while on leave from a secure hospital in south-east London.
Patients wear a device around an ankle when they attend court and when they are allowed out of hospital on authorised visits, some of which are unaccompanied.
Doctors at the Bethlem Royal Hospital in Beckenham say the number of times patients have breached their leave conditions has fallen substantially since the devices were introduced.
The satellite tracking system was introduced after Terrence O'Keefe, a mental health patient, went on the run and strangled a man with his belt in March 2008.
O'Keefe had been held at a secure unit at Lambeth Hospital and escaped while being treated at King's College Hospital in south London.
In June 2009 O'Keefe was found guilty of murdering David Kemp at Great Yarmouth in Norfolk.
The satellite tracking devices have been used by 35 patients at River House, a medium-security psychiatric unit at Bethlem.
The unit treats people with severe mental illness and personality disorders, among them convicted killers and sex offenders.
Professor Tom Fahy, clinical director of forensic services at the South London and Maudsley NHS Foundation Trust, said the device added an extra layer of safety when patients were allowed out on "leave" to prepare them for life in the community.
"The main advantage is that it gives us confidence, about the patient's whereabouts, that they're complying with their leave conditions," said Prof Fahy, a consultant psychiatrist.
"We can... enhance the trusting relationship between the clinical services and the patient."
The device uses three types of technology to pinpoint a patient's location - GPS (global positioning system), GSM (global system for mobile communication) and RF (radio frequency).
It sends signals to a control room which alerts the hospital when a patient goes to an area they're excluded from, or is away for too long.
An accelerometer measures whether the tracking device, and therefore the patient, is moving and at what speed.
The device vibrates to remind a patient when they're nearing the end of their allotted time away from hospital.
"The information is sent from the tracker up to a monitoring station and they can see the information on a map," said Sara Murray, who founded Buddi - the company behind the tracking device - after her daughter disappeared from sight at a supermarket.
"They can actually live-track somebody, so if somebody is missing they can immediately see where they are and follow them."
One of the patients who used the device while at River House told BBC News that it had helped to keep him "more safe".
"If I was to have an accident of any kind it would be easier to pick me up," said the man, who wanted to be known as Edward to safeguard his anonymity.
But Edward, who now lives in a hostel after 13 years in secure hospitals following a conviction for arson, said the tracker was uncomfortable when worn for long periods.
"Sometimes it makes you feel a bit childish... almost labelled," he said.
But Professor Fahy says the satellite tracking system has been successful in cutting the number of times patients have breached their leave conditions.
Doctors also hope the system will cut the costs of searching for patients who have absconded . Each device, including straps, costs £374, plus £50 per month per device for monitoring.
The unit now has 60 devices and plans to use it on more patients over the coming months.