Commuters returning to work have been warned to expect crowd management and social distancing measures as public transport services ramp up.
Rail and Tube services are being increased from today, even though the government has urged workers not to use public transport.
Union officials have complained this is sending mixed messages to millions of workers, with many still wondering whether it is safe to return to their workplace.
The move comes at the start of the first full working week since Boris Johnson set out his plan for easing the coronavirus lockdown in England, urging those employees unable to work from home to return to their places of employment.
The Rail Delivery Group said train services will be increased from around 50% of the standard timetable to 70%.
However, in order to maintain social distancing, their capacity will be reduced to as little as 10% of normal levels, and passengers are being urged to avoid non-essential travel.
Robert Nisbet, of the Rail Delivery Group, said operators faced challenges from increasing travel capacity.
He told Sky News: “We are introducing a number of measures such as enhanced cleaning of trains and maintaining social distancing to make sure our passengers are as safe as possible.
“The advice is only take a train if it’s absolutely necessary but consider other means of transport such as walking or cycling.”
More British Transport Police officers will be deployed to London stations in a bid to control crowds.
Passengers should wear face masks and continue keeping a two-metre distance from other people where possible.
The government is urging transport operators to rearrange, remove or limit seating “to try and ensure social distancing is observed”, which may include blocking off seats in close proximity to others and removing face-to-face seating.
London North Eastern Railway passengers are only allowed to board trains if they hold a reservation as well as a ticket.
Passengers are being asked to sit in a window seat, with one person per row of four seats, and two empty rows between each passenger.
People from the same household will be allowed to sit together but must maintain “a safe distance” from other people.
Avanti West Coast said commuters without a reservation may not be able to travel on their choice of train because of capacities being limited to around a quarter of normal levels.
Train operator Northern said there will be “significantly reduced capacity on each and every one of our trains”.
Rail services have been massively reduced for weeks since the lockdown was announced, causing a collapse in demand and a rise in staff sickness.
Although people in England have been urged to return to their workplace if they cannot work from home, those in Northern Ireland, Scotland and Wales have been told they should still remain at home.
Transport unions say they are concerned the commute back to work is too soon and are telling their members they will be supported if they refuse to work on safety grounds.
The train drivers’ union Aslef is advising its members of their right to refuse to work where they are at risk of serious and imminent danger.
Finn Brennan, Aslef’s organiser on London Underground, said: “Despite our objections, London Underground has insisted that train drivers revert to working as they did before the COVID-19 crisis.
“They are being told that they can no longer continue to work in the safer way that they have been working over the last six weeks.
“This is because the government is insisting that Transport for London maximises the service it operates, regardless of the implications for driver safety.
“Forty-two TfL workers have already lost their lives to this dreadful disease. The government appears to regard them as nothing more than collateral damage.”
The Rail, Maritime and Transport (RMT) union said the increase in train services was a “high-risk strategy”.
It said it had concerns that “rushed political considerations could well override the safety issues for staff and passengers”.
The union has called for new compulsory protections for passengers and rail workers, including the enforcement of two-metre social distancing on trains.
It is also demanding the compulsory wearing of face masks by passengers, which should be provided for free at stations and be able to be disposed of safely.
RMT general secretary Mick Cash said: “We are opposed to the early relaxation of lockdown measures and believe that non-essential workers should avoid using trains. When people absolutely must use a train, there should be new compulsory protections.
“We have the crazy situation of Eurostar passengers arriving with masks on into St Pancras but then not wearing masks when they transfer to the Tube or other rail services.”
A Department for Transport spokesman said the message remained that people should only go to work if they cannot work from home.
He said they should avoid public transport if possible and maintain social distancing if they have no other choice.
The spokesman added: “We have asked operators to increase the number of services from today to help reduce pressure on the transport network, providing more space for social distancing as well as delivering increased reliability and extra capacity for the future.”
Culture Secretary Oliver Dowden told Kay Burley@Breakfast: “It remains the case that people should work from home if they possibly can. Of course if they can’t, they should look to return to work.
“That will reduce the numbers of people on public transport significantly. We’re also saying that people should look to alternative means of transport if they can.
“If people are still using public transport, we have introduced rules about how public transport should be conducted in a way that makes it as safe as possible.”
He added that there “may be some queueing at peak times” in order to ensure social distancing could be adhered to.