More measures are being brought in to protect drivers, following the deaths of at least nine from coronavirus in the UK so far.
These include using signage to discourage people from sitting near drivers and putting extra protection on the screen separating them from passengers.
TfL says it hopes the new boarding method will mean key workers can continue to rely on buses to travel around London.
Travel on the capital’s buses has dropped by 85% since the lockdown was announced.
The trial will be in place from this week on several routes operated by bus firm Abellio out of its depot in Walworth, south-east London, including two that serve hospitals.
Passengers using London buses normally board using the front door and leave using the middle door to smooth the flow of people.
Claire Mann, director of bus operations at TfL, 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.
“We’ve already delivered many other enhanced safety measures and by trialling middle-door only boarding on buses we can gain the information we need to see if we can further improve safety on London’s buses.
“Most Londoners can do their bit to protect our bus colleagues and other critical workers by remembering: stay home, don’t travel, save lives.”
Giving evidence to the Commons’ transport select committee, Mr Heaton-Harris said officials are working to pinpoint areas with a “risk of short supply”.
He added that the Government wants to ensure the equipment is “in the right place at the right time”.
She went on to say “there’s lots of things that can be done” and insisted the Department for Transport is working with bus operators to ensure they “understand exactly what they should be doing”.
This includes having adequate supplies of hand sanitiser, keeping contact between drivers and passengers to a minimum and encouraging the use of contactless payments.