"What we are announcing today is a very substantial change in the way that we want people to live their lives and I can’t remember anything like it in my life time," Johnson, 55, said at a news conference in Number 10 Downing Street, flanked by the government's top scientist and top doctor.
"I don't think there's really been anything like it in peacetime and we have to accept that it's a very considerable psychological, behavioural change that we're asking you, the public, the nation to do," he said.
People should avoid pubs, clubs, restaurants, cinemas and theatres, the government said, though Johnson stopped short of ordering them to close. He said he hoped the venues would take a responsible approach. People should also avoid unnecessary travel and work from home where possible, he said.