Schools across the UK will close from Friday until further notice, except for those of key workers and the most vulnerable.
Mr Johnson did not rule out putting London on lockdown, with coronavirus at its most widespread in the capital.
But he described how a “game-changer” test to determine whether an individual has developed antibodies to tackle the coronavirus is “coming down the track”.
SUBSCRIBE to our YouTube channel for more videos: http://www.youtube.com/skynews
Follow us on Twitter: https://twitter.com/skynews
Like us on Facebook: https://www.facebook.com/skynews
Follow us on Instagram: https://www.instagram.com/skynews
For more content go to http://news.sky.com and download our apps:
Apple: https://itunes.apple.com/gb/app/sky-news/id316391924?mt=8
Android https://play.google.com/store/apps/details?id=com.bskyb.skynews.android&hl=en_GB
Sky News videos are now available in Spanish here/Los video de Sky News están disponibles en español aquí https://www.youtube.com/channel/UCzG5BnqHO8oNlrPDW9CYJog