An investigation is underway

Authorities say the counterfeit versions have A031410 or A041410 written on the bottom of the carton or on the blister strip inside.

Initial tests do not suggest they are a risk to health. But patients have been advised to return them to pharmacies.

Patients with concerns are advised to consult their GP or phone Lilly UK, which makes Cialis, on 0800 085 3847.

"Patients who are in possession of this counterfeit drug should return it immediately," said Professor Kent Woods, chief executive of the Medicines and Healthcare products Regulatory Agency (MHRA).

If patients think that they have taken the product and are concerned about any side effects they should consult their doctor Professor Kent Woods, MHRA

"Whilst initial tests show that the product does not pose an immediate risk to patients, the quality of this product cannot be guaranteed.

"If patients think that they have taken the product and are concerned about any side effects they should consult their doctor."

The counterfeit tablets are understood to have found their way into some pharmacies. Investigations are underway to try to find out how this happened.

Patient complaint

Lilly UK discovered the fake drugs after a patient complained about a crumbling tablet.

The company investigated and found that the tablet was counterfeit. It then contacted the MHRA.

"We want to emphasise that genuine Cialis is not affected by this matter," it said in a statement.

It is believed to be the first time that fake drugs have found their way into the legitimate supply chain in the UK for 10 years.

"This incident highlights the need for the greatest vigilance and scrutiny, especially when medicines are introduced into the system other than from their original manufacturers," said Dr Richard Barker, director general of the Association of the British Pharmaceutical Industry.

"The best protection against counterfeiting is to ensure that the products bought are those supplied by the authentic manufacturer.