The number of people using public transport in Britain's cities could be 20% lower than normal after the end of the coronavirus lockdown.
In London, commuters using buses and tubes could fall by as much as 40% from pre-lockdown levels.
Rail use could drop by 27%, a poll for transport consultants SYSTRA has found.
The survey results capture people's current attitudes about returning to work, but some changes may be carried on into the long term.
The results are bad news for the government, which wants more people to use public transport to cut emissions that are fuelling climate heating.
It could lead to more people driving to work.
It's also challenging for public transport operators, which will face a sharp drop in income until public confidence returns.
Work from home
But the survey offers a glimmer of good news too. It suggests that of those expecting to reduce their use of buses and trains, 24% said they plan to work from home more, which will reduce emissions.
They said they wanted to save on the commute time and cost, and to strike a better work-life balance.
There's a major boost for video-conferencing, too. As many as 67% of people in the 1,500-strong survey said they believe virtual meetings will replace some or all business trips or meetings.
Katie Hall from SYSTRA said: "Our climate emergency has not been cancelled. There is no doubt that this situation has opened up different ways of working for many, but if people start rejecting public transport over the car for work and leisure trips - that's a massive step backwards. Public transport operators must rise to this challenge."
She said public transport operators must work hard to convince commuters that they'll be safe from the virus.
Plan for change
But she also said transport planners would need to think hard about how travel patterns may change permanently after lockdown.
The AA's head Edmund King told BBC News he expected that traffic levels would fall overall. That has implications for the government's £28bn roads programme which is predicated on 1% annual growth in transport demand.
There could also be a boom in walking and cycling in a population that may be more interested in health messages.
The government recently cut red tape on issuing urban road closures to allow councils to exclude cars and create space for walkers and cyclists more easily.
Cycle campaigners want cars excluded from major parts of cities on a permanent basis - which happened recently in Milan.