Sadiq Khan is set to defy ministers and order bus passengers in London to wear face masks.
The mayor said he was considering 'unilateral' action in the wake of the rising death toll among bus drivers.
Transport for London said 34 transport workers with COVID-19 have died - the majority of them drivers employed by bus companies.
City Hall has been lobbying the government to change its guidance and instruct train, tube and bus passengers to wear 'non-medical' face coverings.
Mr Khan told ITV News he was prepared to issue his own edict to London transport users.
If it's the case that despite all our lobbying and the emerging scientific evidence the government doesn't change its guidance, I may well decide unilaterally to instruct Londoners to do that.
Mr Khan's Conservative rival in the postponed mayoral election, Shaun Bailey, said the the mayor did not need government backing.
The mayor doesn't have to ask the government to do it. He can ask them to do that and right now if you provided those masks, I'm sure that the passengers would willingly wear them.