An alert has been issued over thousands of public defibrillators which have an electrical fault that could render them useless.
The fault means devices, which are available in schools, supermarkets, community centres and train stations, may not deliver a potentially life-saving electric shock to someone in cardiac arrest.
Around 2,577 of the Lifepak CR Plus and Lifepak Express automatic external defibrillators are defective, the Medicines and Healthcare products Regulatory Agency (MHRA) said.
An internal component called a reed switch can become stuck in the closed position, the agency said.
The manufacturer Physio-Control has issued a safety alert to people and organisations with the defibrillators, including instructions on what to do.
There have been no reports of failures when treating patients but the MHRA urged owners to check the serial number on their device matches that on the confirmation sheet received from the manufacturer, or the alert on its website.
John Wilkinson, MHRA's director of medical devices, said: "These devices deliver life-saving treatment and it is vital that they work in an emergency.
"Those responsible for them should carry out the instructions by the manufacturer.
"If you have any questions, or have not got the manufacturer's safety alert, call Physio-Control's customer support on 0808 258 0094."
Owners can tell if their defibrillators are faulty if a voice prompt fails to initiate within five seconds, and in those instances the MHRA recommended removing the device and contacting the manufacturer.
The British Heart Foundation said there are more than 30,000 out-of-hospital cardiac arrests in the UK every year.
Maureen Talbot, senior cardiac nurse at the British Heart Foundation, said: "A cardiac arrest happens when your heart stops pumping blood around your body and can be fatal unless you receive the right treatment quickly.
"Every minute without CPR and defibrillation reduces the chances of survival by up to 10%, so it is imperative that these devices are available and ready for use in an emergency.
"If you are concerned about the device in your local community, please do contact the provider for more information."
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