As of 15 June, face coverings are mandatory for most on public transport in England.
Buses, trains and trams are confined spaces where social distancing is almost impossible.
Face coverings may therefore act as an effective barrier against the infected droplets expelled when a coronavirus patient coughs or sneezes.
It is also unclear how many people carry the infection without any symptoms, but are still capable of passing it on.
Enforcing face coverings may help prevent asymptomatic carriers unwittingly transmitting the virus.
Exempt passengers include children, those with disabilities or breathing difficulties, and people who rely on lip reading.
The same rule is not being applied in the rest of the UK.
Officials in Wales, Scotland and Northern Ireland are encouraging passengers to wear a mask or covering, but it is not mandatory.