At the press conference at 10 Downing Street, Boris Johnson told the public they must socially distance themselves from others whenever they leave the house in a bid to slow the spread of the coronavirus.
He warned that fresh air doesn’t provide immunity to the disease as he continued to tell the public they should stay two metres apart when outside.
The Prime Minister said he doesn’t think it takes much imagination to see where the Government ‘might have to go’.
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"We need to think about the kind of measures we've seen elsewhere - other countries that have been forced to bring in restrictions on people's movements altogether, now as I say I don't want to do that,” he said.
"It's so important that that pleasure and that ability is preserved but it can only really be preserved if everybody acts responsibly and conforms with those principles of staying apart from one another and social distancing.
"If we can't do that then I'm afraid we're going to have to bring forward tougher measures."
Mr Johnson, who stood alongside communities secretary Robert Jenrick an deputy chief medical officer Dr Jenny Harries, announced how the NHS had identified 1.5 million people who would be classed as ‘clinically vulnerable’.
Those in the category should expect to receive a letter in the coming days and have been urged not to leave the house for at least 12 weeks.
Mr Jenrick said the Government is creating a network to deliver medicines to the most vulnerable people and discussed how groceries would be delivered by councils, working with supermarkets, by leaving parcels on the doorstep.
He also said there would be opportunities for the public to volunteer for the scheme.
The number of people who have died after testing positive for coronavirus in the UK has risen to 281. The youngest to die in England was just 18.
On March 22, Public Health England said there are 5,683 confirmed cases – a rise of more than 600 in 24 hours.