Sex enhancement tablets that can be dangerous if taken without medical advice are being sold in Asian corner shops, trading officials warn.
Thousands of cheap variations of the licensed drug Viagra have flooded into Britain from India.
Trading standards officials warn that untested imitation pills could have fatal consequences.
Dubbed the "pills that kill", anyone who has bought them is being urged to throw them away immediately.
The potentially deadly pills being sold under the counter include Vigora, Proxyron and Kamagra. Kamagra oral jelly is also being sold.
Viagra is licensed in Britain to treat erectile dysfunction and is only available with a doctor's prescription or from some pharmacies.
Health experts say just one of these pills could kill someone who is already taking prescribed medicines for liver, heart, kidney or blood pressure problems.
"These tablets are illegal and unlicensed and have not been tested to UK standards to ensure they are safe. Anyone taking them could be risking their life," said Sangeeta Sharma, a senior pharmaceutical advisor who works for the local health authority in Ealing, West London.
In recent days trading standards officers working undercover have seized nearly 1,600 tablets from one shop in Southall, the same Ealing community where 2,000 tablets were seized three weeks ago.
Authorities believe the fake pills are also being sold in Yorkshire and Scotland and they say they are now targeting the importers who bring in the drugs from the subcontinent.
In India a strip of four tablets can be as cheap as 50p, compared with the ?15 they fetch on the black market in London.
At Boots the chemist, which began selling Viagra without prescription in February 2007, an hour-long consultation and medical tests conducted by a pharmacist cost ?50, which includes four tablets.
The manufacturers of one of the drugs in question have expressed their concern over the illegal trade, saying their drug should be taken only under medical supervision.
"As far as Ajanta is concerned people should always seek medical advice," said Arvind Agrawal, from Ajanta Pharma Ltd, the makers of Kamagra. ( BBC )