“The objective is to slow the spread of the virus and we judge that this is the moment to do that,” he said.
Asked how long the shutdown could last, Johnson responded: “I wish I could give you an answer about how long it will be.”
He urged parents not to leave children with groups most vulnerable to COVID-19 like the elderly or chronically ill.
“And that’s going to be difficult too and I want to thank families for their sacrifice at this difficult time,” he said.
Children of key emergency services workers could continue to attend certain schools in a bid to keep their parents at work during the escalating emergency.
In Australia, Prime Minister Scott Morrison has pushed back against calls to close schools, citing modelling that the measure could starve hospitals of essential workers.
“The disruption that would occur from closures around this country would be severe,” he told reporters at Parliament House on Wednesday.
“What do I mean by severe? Tens of thousands of jobs could be lost, if not more. The impact on the availability of health workers? A 30 per cent impact on the availability of health workers.”
In a press conference at the White House, the US coronavirus response coordinator Deborah Birx said officials there were concerned about new information coming from France and Italy which suggested more young adults were being infected than first thought.
The number of official cases in Britain stood at 2626 on Thursday morning, however the chief scientific adviser has said the true figure is likely about 55,000.
The Italian government on Wednesday revealed the largest single-daily death toll of any nation since the outbreak began in China, with 475 new deaths bringing Italy’s total to 2978.
The overwhelming number of infections in Britain are in London and Johnson did not deny growing speculation that a lockdown of the capital was imminent.
The government has asked people to avoid non-essential contact or travel but has resisted ordering shops, restaurants and pubs to close as most other European nations have.
“Of course people must take their own decisions, I am a believer in freedom. But let’s be in absolutely no doubt that these are very important choices we are making in our everyday lives,” Johnson said.
“And the more closely, the more strictly, the more ruthlessly we can take the advice… then the better we will be able to protect our National Health Service, the fewer deaths we will have, and the less suffering there will be in the UK population, and the faster we will get through this and the better we will bounce back eventually.
“As I say, absolutely we do not rule out…taking further and faster measures in due course.”
Bevan Shields is the Europe correspondent for The Sydney Morning Herald and The Age.