British Prime Minister Boris Johnson was disastrously slow to impose a lockdown in 2020 because he thought Covid-19 was a scare story and even considered getting injected with the coronavirus on live television to show it was benign, his former chief adviser said.

Johnson subsequently caught Covid-19 early in the pandemic and was so ill that he was moved to intensive care at a London hospital where he received litres of oxygen. He later said plans had been prepared to announce his death...