Professor Chris Whitty explains why there needs to be a pause between each stage easing the lockdown.
Boris Johnson has been defending his four-stage plan to ease the coronavirus restrictions in the hope of lifting all social contact limits by 21 June at the earliest.
Schools will open on 8 March followed by non-essential shops, hairdressers, and gyms in mid-April. Then outdoor areas of pubs and restaurants can follow suit and if infection rates continue to fall indoor seating will be allowed from mid-May.
Mr Whitty, the chief medical officer, explained why each stage would be measured in four-week periods.