International regulators have launched a series of “sting” operations against illegal producers and sellers of the flu drug Tamiflu, as US, Dutch and Austrian officials trace a growing volume of counterfeit tablets back to Asia.

The UK’s Medicines and Healthcare Products Regulatory Agency (MHRA) said on Tuesday it had found nearly 20 websites around the world illegally selling Tamiflu. It had placed orders with several so it could analyse the drugs on offer and begin prosecutions of those involved.