London Underground commuters have been warned that if they do not stop packing into trains more people will die from coronavirus.
Mayor of London Sadiq Khan issued the alert after photos showed crowds in Tube carriages for a second morning running.
Mr Khan insisted the network was running for essential workers only.
A transport union called for police officers to be deployed at major stations to stop overcrowding.
More than 130 people have died in London from Covid-19 - a third of the whole of the UK.
But the trains were still crowded during rush hour despite Boris Johnson putting the UK on lockdown.
Mr Khan demanded employers enable their staff to work from home "unless it's absolutely necessary".
"Ignoring these rules means more lives lost," he warned.
Transport Secretary Grant Shapps said he spoke to Mr Khan about ensuring there is "enough space to be safe" on London Underground trains for those who must travel, and offered the support of his department.
Nurse Julia Harris, who commutes to work at Imperial College NHS Trust, said she had left earlier and changed her route to avoid crowds but still found services busy.
"Seats on the train all had at least one person so people needed to stand, and the District line was busy as well," she said.
"I still don't think things have improved as a large amount of people are commuting early in the morning.
"It is concerning because I have to come to work.
"The choice isn't there and my commute is quite long. I worry for my health more on my commute than actually being in the hospital."
Nicola Smith, who works at a central London hospital, said "I'm risking my health."
She called for either Mr Khan to reverse his decision to reduce Tube timetables or the prime minister to "start policing who's getting on".
Manuel Cortes, general secretary of the Transport Salaried Staffs' Association (TSSA), said: "Sadly, the situation on the London Underground has not improved.
"We urgently need British Transport Police and other officers at major stations across London's transport network to ensure only those with a valid reason to travel are doing so in this emergency."
Finn Brennan, of the train drivers' union Aslef, said he had received reports from Tube staff that "there were a large number of construction workers" on the Underground.
He said: "Unless the government act with complete clarity to close down construction sites, then the tube will not.
"There are too many employers that are just not taking this seriously. We are trying our best to make everyone listen, but the situation is stark.
"It cannot go on the way it is or the Tube will not be able to continue. But, let me be clear, we very, very much want it to continue as it is vital."
By BBC London's transport correspondent Tom Edwards
Public transport is currently meant to be for "critical workers" only but it doesn't seem to be working.
Union reps on the tube have been left apoplectic for a second day because lines have been very very busy, particularly at the start of the service.
Footage I have seen shows passengers crammed in next to each other and there is now talk among members of staff who are union members of a walkout - they are so unhappy with the situation.
They are particularly picking out construction workers and believe all the sites in London should be closed. They want the government to put forward a financial package for these self-employed workers.
There are also calls for ID checks at stations or the definition of key worker to be tightened.
In the strongest wording I have ever seen - Transport for London says fewer people need to travel or people will die.