Transport for London (TfL) could close up to 40 tube stations across the city to help slow the spread of the coronavirus.
Some closures came into effect on this morning, with a reduced service to run from Friday.
Waterloo & City line will not operate from Friday, with TfL adding there will be no night services and buses will be reduced.
The tighter measures have been announced after Prime Minister Boris Johnson said coronavirus was spreading faster in London than the rest of the UK.
Official figures from Public Health England show 953 cases recorded in London compared to a total across the UK of 2,626.
And the proportion of deaths is higher in London than elsewhere in the country, with reports putting the capital's death toll at 35.
Mayor of London Sadiq Khan said those living in the city should not be travelling unless it was essential.
He said: "People should not be travelling, by any means, unless they really, really have to.
"Londoners should be avoiding social interaction unless absolutely necessary, and that means they should be avoiding using the transport network unless absolutely necessary.
"London will get through these extraordinarily challenging times, and ensuring the capital's critical workers can move around the city will be crucial.
"Frontline staff across our health and care service - as well as those ensuring Londoners stay safe and can access food and other essentials - should be commended for their hard work.
"We owe it to them to do whatever we can to help them do their jobs effectively.
"I'm urging Londoners to only use public transport for essential journeys. Everyone should follow this and the other advice to help keep themselves and each other safe."
All nine Tube stations that have been closed:
Circle Line: Barbican.
District Line: Bow Road.
Northern Line: Borough; Goodge Street; Hampstead.
Piccadilly Line: Arsenal; Bounds Green; Covent Garden; Manor House.
All 31 Tube stations that could close:
Bakerloo line : Lambeth North; Regents Park; Warwick Avenue; Kilburn Park; Charing Cross.
Central line : Holland Park; Queensway; Lancaster Gate; Chancery Lane; Redbridge.
Circle line : Bayswater; Great Portland Street.
District line : Stepney Green; Mansion House; Temple; St James's Park; Gloucester Road.
Jubilee line : Swiss Cottage; St John's Wood; Bermondsey; Southwark.
Northern line : Tuffnell Park; Chalk Farm; Mornington Crescent; Clapham South; Tooting Bec; South; Wimbledon.
Piccadilly line : Caledonian Road; Hyde Park Corner.
