The medicines watchdog, the MHRA, is urging people with diabetes to check their insulin pens.
It says a manufacturing fault has led to some batches of pens or cartridges containing the wrong amount of insulin.
The products affected are the NovoMix 30 Flexpen 100U/ml pre-filled pen and the Novomix 30 Penfill 100U/ml cartridge.
Last year there were just over a million prescriptions for these products dispensed in England.
They are used mainly by children with type 1 diabetes, to regulate blood-glucose levels, although some people with type 2 diabetes also use the pens.
The MHRA warning has also been circulated by the Department of Health in Northern Ireland.
The Medicines and Healthcare products Regulatory Agency says patients with pens affected by the recall should consult their GP or nurse to arrange for a new supply as soon as possible.
In the meantime patients should not stop their treatment, it said.
The Agency has issued a list of batch numbers printed on the pen or cartridge. It says only a small fraction of the cartridges sold are likely to have been affected.
For the full list of batch numbers visit: www.mhra.gov.uk