"Coronavirus face masks" offered by third party sellers on Amazon.

An investigation has been launched into a retailer of “coronavirus face masks” being sold via Amazon, Yahoo News UK can reveal.

The online retail giant has been forced to remove a face mask product which was ranked as the number one best-seller on its health and personal care category, after it was flagged by Yahoo News UK.

On Thursday, Amazon confirmed that the product had been removed from the site.

The Advertising Standards Authority (ASA) also confirmed it has launched an investigation into one of the third-party sellers. It made it clear it is not investigating Amazon.

It is not known how many of the masks have been sold as Amazon does not give out third party sales figures.

Face masks have become an increasingly common sight on public transport as the coronavirus continues to spread around the world. The virus has killed more than 500 people in China and in excess of 20,000 cases have been confirmed worldwide.

On Thursday, the UK had its third coronavirus case confirmed.

Yahoo News UK identified a number of products that identify or imply that they were safe to use to protect against the virus.

One product, which is no longer listed, sold for £210 and had the following description: CORONAVIRUS SAFE 3M Aura Flat Fold Face Mask Disposable Dust, Mist, Fume, Respirator, FFP3, Valved, 9332+ (10 pieces)

The best seller, which was removed, said: 5X Flu Medical Face Mask | Virus | Coronavirus | SARS | Norovirus | Protected in Sealed Bags | UK Stock

Public Health England has cast doubt on the effectiveness of face masks outside of clinical settings, with one medical expert telling Yahoo News UK there was “little evidence of widespread benefit”.

This mask had "coronavirus" in its product title.

The same mask ranked top in Amazon UK's health and personal care category. It has since been removed.

Many of the “coronavirus” masks sold on the site, including the health and personal care bestseller, provide no information as to whether the masks actually stop the wearer catching or spreading the disease.

When Amazon was contacted by Yahoo News UK on Wednesday about the bestselling face mask, the site took down the product listing within 24 hours.

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Amazon is also understood to be taking action against the vendor of the bestselling mask, which can no longer be bought on the site.

A spokesman for Amazon said: “We work hard to build a great experience for our customers and sellers and take action to protect them from those that threaten their experience in our store.

“We have systems and processes to detect suspicious behaviour and we have teams that investigate and take action quickly.”

The mask marked with "Coronavirus Safe" in its product title.

Public Health England said face masks are only of use if worn and removed correctly, frequently changed, safely disposed and used with “good universal hygiene behaviour” - which tend to be practised less by wearers the longer they wear a mask.

Dr Jake Dunning, head of emerging infections and zoonoses at Public Health England, said: “People concerned about the transmission of infectious diseases should prioritise good personal, respiratory and hand hygiene.

“There is very little evidence of widespread benefit from their (face mask) use outside of these clinical settings.”

Paul Hunter, professor of health protection at the University of East Anglia, said: “The advice is at the moment, for people in the UK, the risk is so low that you don’t really need to be that concerned at present.

“Whether than changes in future, time will tell, but I would advise not to buy and start wearing face masks.”

He warned that people wearing face masks should not believe they are protected and said he is reviewing the evidence for the effectiveness of face masks but is unsure what the findings will be.

However, the US’s Center for Disease Control and Prevention recommends face masks for people confirmed or suspected of having coronavirus when around other people.

The ASA told Yahoo News UK on Thursday it has launched an investigation into one of the sellers that listed its product on Amazon.

A spokesman for the ASA said: “Following an enquiry on the products (sent by Yahoo News UK)... I can confirm that we are going to initiate an investigation against one of the third party sellers over whether they have breached our social responsibility advertising rules.”

Coronaviruses on average are just over 0.1 micron (0.0001mm) in dimension, and many masks are unable to filter particles that size.

“The virus itself is smaller than the pore size of these masks,” Prof Hunter said.

“However, single viruses are not generally the way these things spread.

“They are spread in aerosols of spit, snot and sputum and those particles are much bigger than the virus itself.”