Wearing face masks while travelling in London should be compulsory, city mayor Sadiq Khan has told the government.
Despite UK public health experts not currently recommending the use of face-coverings, Mr Khan is lobbying for guidelines to be changed.
It comes after it was made compulsory in New York on Wednesday, with similar schemes also being operated in the Czech Republic and Slovakia.
UK government social distancing guidelines do not mention face masks.
"[But] the evidence around the world is that this is effective," Mr Khan said.
"I'm lobbying our government and advisers to change their advice, and I want us to do that sooner rather than later.
"They are already reviewing this on the basis of our representation."
The World Health Organization (WHO) says it remains the case that medical masks should be reserved for healthcare workers, not the general public.
But WHO special envoy Dr David Nabarro has suggested more widespread use of masks will become "the norm" as the world adjusts to living with Covid-19.
Shaun Bailey, the Conservative candidate for London mayor however, has accused Mr Khan of not doing enough to help supply personal protective equipment (PPE) to Transport for London (TFL) staff and this also should be compulsory.
Mr Bailey claims to have sourced 600,000 face masks and vinyl gloves - enough to protect every bus worker for 30 days.
"The mayor of London is also the chairman of TfL, and his failure to provide transport workers with PPE is putting lives at risk," said Mr Bailey.
"If he wanted to, he can start tomorrow by sourcing PPE for all 60,000 of the transport staff who work for him.
"Now more than ever, London needs its mayor to take responsibility and to stop blaming the government to score political points during a national crisis."