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”.
Coronavirus in London: Latest updates
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 the deaths of drivers “have got to stop” and “more action is urgently needed”.