People with diabetes are being urged to check their blood glucose meters after a potentially dangerous fault was identified in two models.
The Medicines and Healthcare products Regulatory Agency (MHRA) said around 22,000 people use the One Touch Verio Pro and One Touch Verio IQ meters.
Both models, used across the UK, are manufactured by LifeScan.
The MHRA said the Pro gives falsely low readings and the IQ turns off if a person has an extremely high reading,
The problem also affects meters used by health professionals in hospitals and other settings. An alert has been sent out to them as well.
The MHRA warned the software fault could mean diagnosis and treatment are delayed.
Medical devices director John Wilkinson said: "Extremely high blood glucose levels in people are rare, but if they do occur the person will require immediate medical attention."
The faulty Pro model has serial numbers that start with RA, RB, RC, RD and RE, while the IQ numbers start with TA or TB.
People who have the meters are being asked to contact LifeScan on 0800 279 9118 and it will be replaced free of charge.