Mayor Sadiq Khan, whose father was a bus driver, led tributes to the victims as authorities said passengers would no longer be able to use the front door near the driver.
“It breaks my heart that 20 London bus workers have lost their lives to #Covid19. It could easily have been my dad and his friends,” Khan wrote on Twitter.
“Our transport workers are heroes and we must do everything we can to protect them.”
He joined a minute's silence called by the trade union Unite, which has been pressing for better protection for London's 20,000 bus drivers and other transport staff.
London is on the front line of the coronavirus outbreak in Britain, where almost 14,000 people have died, making it one of the worst hit countries in the world.
Since a nationwide lockdown was introduced last month, bus passengers numbers in London have fallen by about 85 per cent.
But a reduced service is still running on the buses and trains for key workers. Four Underground and train staff have also died.
Transport for London (TfL), the city's travel authority, has already introduced a new cleaning regime for buses, stations and depots.
The seats at the front of buses closest to the driver have been roped off and drivers sit behind a clear protective screen.
In the new measure to be introduced from Monday, the front doors will now be kept shut with passengers asked to get on and off using the middle doors.
Pete Kavanagh, Unite's London representative, said it was a “very welcome move”.
“We have lost members of our bus family in recent days and we refuse to lose any more,” he said.
“Unite has been asking for central boarding as an essential safety measure during these times because bus workers are, understandably, fearful for their health.”
Not all protective measures have been so well-received by workers.
The RMT union reported that one bus operator in south-west England responded to its demands for more protection by fitting “what can best be described as a shower curtain” to the driver's cab.
Drivers complained that “the curtain is so flimsy members of the public have been pulling it to one side in order to speak to the driver” — which risks greater contact with passengers.
Khan is also lobbying the British government to advise people to wear face masks on public transport, to try to stem the spread of coronavirus.
Ministers have expressed scepticism about the idea in the past, but say the issue is under review.