A criminal investigation has been launched after a counterfeit batch of drugs used in the treatment of prostate cancer was discovered.
The pills concerned carry the batch number 65520
The batch claiming to be of the hormone treatment Casodex has been recalled.
The Medicines and Healthcare products Regulatory Agency (MHRA) said: "We take this very seriously and a criminal investigation is being carried out."
Patients whose medicine carries the batch number 65520 have been urged to contact their pharmacist.
The MHRA, which oversees the regulation of medicines, said it had been contacted by a wholesaler who had been offered a suspicious batch of the medicine.
Tests showed the batch medicine contained only 75% of the amount of the active ingredient bicalutamide shown on the label.
"At present there is no evidence of patients having any adverse reactions specifically related to the counterfeits," the MHRA said in a statement.
Casodex is produced by the pharmaceutical company AstraZeneca.
Dr Chris Hiley, head of policy and research at The Prostate Cancer Charity, said: "This is the first time we have heard of counterfeit supplies of a well known prostate cancer treatment arriving and, possibly, being dispensed in the UK through the 'legitimate' supply chain.
"This is clearly of great concern. The wholesaler who alerted the MHRA should be commended.
"We have to wait and see how many of the batch are located and whether in fact men have taken any of them and, if so, for how long."