Police will be available to stop non-essential workers from travelling on the London Underground, the force said.
There is growing concern about rush hour Tube carriages remaining packed despite orders for only key workers to use public transport to slow the spread of the coronavirus.
A British Transport Police spokeswoman said the force recognises it has “an important role to play with our partners in helping ensure that only those making essential journeys for work are using the Tube and rail network”.
She added: “Our officers will be on hand to support rail operators if people are clearly disregarding the advice.”
Boris Johnson has raised concerns about cutbacks in London Underground services with the capital’s mayor Sadiq Khan.
The Prime Minister’s official spokesman said both Mr Johnson and Transport Secretary Grant Shapps had spoken to the mayor.
The spokesman said: “The Prime Minister raised with the mayor the issue of reduced services on the Tube and its impact on people trying to get to work. But this is a matter for TfL (Transport for London).
“The Transport Secretary has also spoken to the Mayor of London on this issue where they discussed looking at ways to make sure appropriate timetabling is in place to ensure it is safe for those who need to get to work because they cannot do this from home.”
Mr Shapps has ordered the rail industry to ask retired staff including signallers to “assist in this great national effort to beat coronavirus”.
TfL – which is chaired by Mr Khan – has suspended the Circle line and Waterloo & City line, and reduced frequencies on other parts of the Tube network.
Bus services have also been cut.
Mr Khan said “growing numbers” of TfL staff are off sick or self-isolating, which means “we cannot run more services than we currently are”.
He added that many people still travelling are on zero-hours contracts, work in the gig economy or are freelancers.
A “proper package of support” from the Government for these workers “would alleviate this situation”, the mayor added.
Nurse Julia Harris, who commutes to work at Imperial College NHS Trust, said she had left earlier and changed her route in a bid to avoid crowds but still found services busy.
She told the PA news agency: “Seats on the train all had at least one person so people needed to stand, and the District line was busy as well. 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.”
I feel like I am risking my health trying to get to workDanielle Tiplady, nurse
Nicola Smith, who works at a central London hospital, said she was “risking my health” to commute by Tube because it was so busy.
She called on TfL to increase services or the Prime Minister to “start policing who’s getting on”.
And she added: “Help me!”