UK Prime Minister Boris Johnson has returned to work in Downing Street two weeks after recovering from COVID-19.

Johnson delivered a statement to reporters outside 10 Downing Street warning against easing the lockdown too soon, saying he did not want to "throw away the efforts and sacrifice" made so far by the British people.

"Everyday I know that this virus brings new sadness and mourning to households across the land and it is still true that this is the biggest single challenge this country has faced since the war," Johnson said.

He likened COVID-19, which led to him spending time in intensive care, to an "invisible mugger" and said he could not yet set out a date for an end to the lockdown. However, preparations for "winning phase two" are under way, he added.