Dubbed the 'pills that kill', anyone who has bought them is being urged to throw them away immediately, the British Broadcasting Corporation reported, adding the drugs being sold under the counter include Vigora, Proxyron and Kamagra and Kamagra oral jelly.
Viagra can only be sold with a doctor's prescription or at select pharmacies in Britain.
Health experts say just one of the generic pills could kill someone who is already taking medicines for liver, heart, kidney or blood pressure problems. Asians in Britain - prone to heart conditions and diabetes - are said to be more at risk.
"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," the BBC quoted Sangeeta Sharma, a senior local government pharmaceutical advisor in West London, as saying.
It said trading standards officers working undercover have recently seized nearly 1,600 tablets from one shop in Southall and 2,000 tablets in nearby Ealing, and believe the drugs are also being sold in Yorkshire and Scotland, in northern Britain.
The west London suburbs of both Southall and Ealing have large populations of Asians.
In India, a strip of four tablets can be as cheap as 50 pence, compared with 15 pounds in London.
Boots, a well-known British pharmaceutical chain, sells Viagra pills over the counter, but only after an hour-long consultation and medical tests conducted by a pharmacist. It sells four tablets for 50 pounds.