Passengers traveling on public transport in London are welcome to wear a face cover.
Transport for London (TfL) said the measure could prevent people with coronavirus from infecting others.
The announcement concerns passengers using the London Underground network, buses, black taxis and minicabs.
Some transport users already covered their faces, but it was not compulsory.
TfL said in a statement, “Please travel outside of rush hours and use a face covering, wear hand sanitizer and wash your hands before and after your trip.
“The last piece of advice from the government is that, although face covers are unlikely to keep you from getting the virus, they could keep you from spreading it to others.
“Face covers are particularly important when the social distance of two meters is difficult to maintain.”