TfL commissioner Andy Byford is expected to make mask wearing compulsory on the underground, bus and rail.
People who refuse to wear a mask can be turned away from the transport service but they will not face fines or prosecutions after July 19.
It comes after national rail and bus operators in England said they would only ask passengers to follow government guidance.
Unions want face coverings to stay but say that staff cannot make sure people comply without police intervention.
However, mask wearing will remain compulsory on London transport services, The Guardian reported.
The mayor of London Sadiq Khan has already said he wants masks to remain on transport.
“I’m sure Londoners will continue to do the right thing as they have done throughout the pandemic and continue to wear a face covering on TfL services,” he added.
Some train operators have said they will ask people to wear masks when it is busy.
A spokesperson for the Rail Delivery Group, which represents the industry, said: “Passengers should follow the government guidance and, out of respect for others, wear face coverings if an indoor setting is busy. Train travel is low risk, with the majority of carriages well ventilated by air conditioning systems or by doors and windows.”
Boris Johnson said mandatory face mask wearing ends on July 19.
However, the prime minister urged everyone to wear their face coverings in “crowded places”.
The new guidance has been criticised by experts and transport operators who say cross-industry policy is key to preventing confusion among passengers.
Further complicating matters, other parts of the UK are not adopting the same stance.
The Scottish government announced that mask-wearing would remain compulsory in indoor settings after 19 July.
In Wales, face coverings also remain mandatory in all indoor public places while mask-wearing rules are due to be eased in Northern Ireland on July 26.
Airlines British Airways, Ryanair and easyJet have said they will continue to require face masks from Monday alongside the Eurostar service.