Commuters in London said social distancing was “next to impossible” as many made their first journeys to work since lockdown rules were eased.
People in England are being encouraged to return to work if they cannot work from home.
The government said it would have to “take steps” if too many people used public transport.
One commuter said most people were not wearing masks, leaving him fearing “a second wave of infection”.
“It is next to impossible to social distance on the Tube,” Matt Hickson told the BBC.
The 47-year-old street works inspector said he saw “less than 10% of commuters wearing masks” on his London Underground journey.
“People are taking liberties not only with their own health but with other people’s.
“There could be a second wave of infections coming.”
Transport for London (TfL) said passenger numbers between 05.00 and 06.00 BST were up 9% compared to the same time last week.
“Any notion of self-distancing on buses or tubes is going to be impossible,” said Drew Aspland.
The 36-year-old walked to work in central London from Bethnal Green after seeing his bus “back to pre-lockdown levels of passengers”.