A severely reduced number of passengers will be allowed to use the Tube network in order to stick to strict social distancing rules, a leaked Transport for London (TfL) report has warned.
Its message is extremely stark - with social distancing it will only be able to transport tens of thousands of commuters rather than hundreds of thousands.
The report, seen by BBC London, shows that even with a 100% service and two-metre social distancing, only 50,000 passengers could board every 15 minutes. That compares to 325,000 normally boarding every 15 minutes at the peak of rush hour prior to lockdown.
Even with a reduced one metre social distancing and a full service, the Tube could handle only 80,000 passengers boarding every 15 minutes.
The current service would be able to handle 30,000 passengers boarding every quarter of an hour.
London mayor Sadiq Khan told the Metro newspaper on Tuesday he was "really worried" people using public transport will be at risk of infection if the lockdown eased.
A TfL spokesman told BBC London: "Whatever happens over the coming weeks and months, everyone who can work from home must continue to do so for some time yet.
"Our intention is to progressively build up service levels, but it is clear life simply won't be returning to what it was before.
"We are working with Government to understand how the restrictions on travel may be lifted - including their expectations on social distancing. This will dictate how many people can be safely carried alongside extensive mitigating measures like re-timing journeys to spread demand out of peak times and managing stations differently."
The spokesman said when offices reopen there would be "significant" challenges in enabling those who cannot work from home return to their desks and socially distance themselves.
"We expect that there will be an enormous travel demand challenge that we will all need to overcome together, and planning is underway on how to meet that challenge and help London get moving again, safely and sustainably."
Sources say the report shows how seriously social distancing will limit Tube capacity and how difficult it will be to deal with large numbers of passengers.
London's transport bosses are grappling with how lifting the lockdown will affect services and are facing unprecedented challenges on many fronts.
TfL is currently in negotiations with government for financial assistance and has said previously a huge campaign to get passengers to stagger journeys would be needed.
In other major world cities with metro systems masks have been made compulsory for passengers so that may well be an option in London to mitigate risks to public health and it is strongly supported by Sadiq Khan.