The Delta variant of Covid-19's reproduction rate could rise as high as seven, the chief medical adviser for NHS Test and Trace has warned MPs.

The comments from Dr Susan Hopkins, who is also deputy director of Public Health England's National Infection Service, follow a delay to the last step of the roadmap out of lockdown, which was pushed back from 21 June to 19 July earlier this week, as the new strain of the virus spreads across the UK.