NHS staff have been asked to shave their beards to allow masks to fit more securely in a bid to limit the spread of coronavirus.
Southampton University NHS Trust sent a mass email to workers to tackle the "known problem" of ill-fitting masks on bearded faces.
Medical director Derek Sanderman attached an image from the United State's Centres for Disease Control and Prevention (CDC) showing 36 different kinds of facial hair and whether they can fit inside a mask.
"You will see that the presence of facial hair compromises the ability to protect any individual through a mask," the email said.
"I am writing to ask those who do not have a strong cultural or religious reason for a beard and who are working in at-risk areas to consider shaving."
The detailed diagram states small moustaches such as the 'Zorro' and 'Painter's Brush' will fit under the mask, but beards like the 'Garibaldi' and the 'French Fork' are unacceptable.
Mr Sandeman acknowledged shaving a beard could be a "big ask", as "beards are so popular at present."
"However I believe this is the right thing to do," the email said.
Public Health England and NHS England have been contacted to see if the guidance will be issued to hospitals nationally or if it is a matter for individual trusts.
The trust said the CDC diagram had been widely shared by infection experts on social media, but did not relate specifically to the coronavirus.
According to the Health and Safety Executive, stubble and beards “make it impossible to get a good seal of the mask to the face”.
“Many masks rely on a good seal against the face so that when you breathe air in, it is drawn into the filter material where the air is cleaned. If there are any gaps around the edges of the mask, ‘dirty’ air will pass through these gaps and into your lungs," its website reads.