The government has advised people not to go to pubs, restaurants, cinemas or restaurants to help stop the spread of coronavirus.
Asked whether tougher measures were needed to shut down London where bars, public transport and businesses remained busy, Johnson said: "We've always said we're going to do the right measures at the right time.
"A lot of people are making a real heroic effort to comply with the advice we've given, but as I've said ... we keep everything under continuous review and we will not hesitate to bring forward further and faster measures where that is necessary."