London (CNN) The UK is banning people from leaving their homes apart from for a few "very limited" reasons, Prime Minister Boris Johnson announced on Monday, as he imposed the most stringent restrictions seen in Britain since the end of World War II.

Police will break up public gatherings and fine people flouting rules, Johnson said in a remarkable televised statement on Monday evening, dramatically ramping up the country's response to the growing coronavirus pandemic.

Shops, gyms, and places of worship will all close, and social events including weddings -- but not funerals -- will be postponed.

"From this evening I must give the British people a very simple instruction -- you must stay at home," Johnson said. "Because the critical thing we must do is stop the disease spreading between households."

"People will only be allowed to leave their home for the following very limited purposes," he added, listing four reasons for which citizens can go outside; shopping for basic necessities, doing one form of exercise a day, providing medical services, or going to work if it is absolutely vital.

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