London's transport system may be unable to cope with a surge in demand when lockdown measures are lifted, a briefing to emergency planners says.
The report, seen by the BBC, says the Tube would be "rapidly overwhelmed" if social distancing were maintained.
It further warns of added pressure on police, who may be called upon to manage crowds and enforce restrictions.
The London Strategic Coordination Group said the document should "not be seen as what will happen".
It contains a number of planning scenarios based on current social distancing measures, a spokesperson said.
Capacity 'down to 15%'
The "lockdown release" briefing sets out the daunting issues facing agencies involved in keeping London operational.
Emergency planners got little warning of the lockdown being brought in and are concerned they will not have time to prepare for the challenges brought by relaxing restrictions.
London's transport network, which has 7,000 furloughed staff, would need three to four weeks to prepare, according to the briefing.
A government review on whether to "unlock" the lockdown is due by 7 May.
The briefing warns that maintaining a 2m (6ft 6in) social distance would reduce the capacity of the London Underground to 15% of normal levels, and buses to 12%.
Currently, buses are at full capacity during busier times, with social distancing in place.
The briefing also says traffic congestion could increase if people opt to use their cars, and it concludes there would be "no ability to get all children to school by bus".
Crime to re-emerge
Another big issue identified in the report is crowd management.
The police may need to keep order and managing compliance with social distancing alongside increased queues and crowds would put officers "under significant pressure", the briefing says.
In addition, the report says, crime will re-emerge as people return to something closer to normal life.
Decisions are needed on a school opening strategy, and on staggering or phasing commercial activity.
But first planners need to know what level of social distancing the government will demand, and whether it will require or advise the wearing of masks.
Surges in demand for personal protective equipment (PPE) are expected, possibly resulting from media coverage about wearing masks in public.
The co-ordination group's spokesperson said it was "important that emergency planners understand all the implications and the impact that any changes to lockdown measures could have on individual organisations and the wider public".
Planning going on in London will be replicated around the country as authorities prepare for what is being informally referred to as the "unlock".
Officially, the ending of the lockdown moves planning to the "recovery phase" of the crisis, as the country gets to grips with "the new normal" of living with Covid-19.