Passengers using some London buses will only be able to board through the middle doors as part of increased efforts to protect drivers.
Nine bus workers have died in the capital after contracting coronavirus since the outbreak began.
Transport for London (TfL) said the four-week trial using middle-door only boarding was in addition to measures such as enhanced cleaning.
Unions want more action as some drivers said protection was “inadequate”.
A total of 14 public transport workers have died in the capital after contracting Covid-19.
London’s Mayor Sadiq Khan has called the deaths “devastating”, adding that it was “really important we treat public transport workers as heroes”.
When asked whether there had been a failure to protect drivers when the virus first emerged, Mr Khan said it was “a question I ask myself and my top team all the time”.
“We’ve made sure we’ve got the most safety measures we can possibly take in London.”
However, the Unite union, which represents 20,000 London bus workers, said “more action is urgently needed”.
The middle-door boarding trial is taking place on several Abellio routes that operate out of Walworth bus garage, including two which serve hospitals.
Most buses in London have both front and middle doors, with passengers usually using the front ones for boarding.
TfL said the trial would allow them to improve social distancing for drivers while seeing “how the change works in live operations and whether it causes any issues”.
Other safety measures being used across the network include introducing signs to discourage people from sitting near the driver and adding an extra layer of protection to the clear screen that separates the driver from passengers.
Anti-viral disinfectant is also being used to clean the interiors of vehicles.
TfL’s director of bus operations, Claire Mann, said “London’s hard-working transport workers are making a heroic effort at the frontline of the fight against this pandemic, and it is only right we consider everything we can to protect them.”
TfL said it had worked with the Unite union and bus operators to improve safety for workers, but unions have called for further action.
Regional secretary Pete Kavanagh said the transport authority “needs to instruct all bus companies to lock front doors with passengers no longer entering the bus next to the driver with immediate effect.
“To improve social distancing, which will protect drivers and passengers alike, the maximum number of bus passengers must also be reduced,” he said.
The number of people using buses in the capital has fallen by about 85% compared to this time last year.
Transport bosses have said they have been “encouraged” by the fall in passengers and have called for those who “really have to go to work” to try to avoid the rush hours.