London: You couldn't make it up. In a single extraordinary day, three of the four most important figures leading Britain's fight against coronavirus have themselves been hit by the virus.

First, the big one: Boris Johnson. The Prime Minister developed a mild temperature and persistent dry cough on Thursday afternoon and within hours had been secretly tested by staff from the National Health Service. The word came through to Downing Street around midnight that the result was positive, making him the first world leader to be directly affected by the pandemic.

Prime Minister Boris Johnson is in self-isolation after testing positive to coronavirus. Credit:AP

Johnson used a hastily filmed video to announce the bombshell diagnosis on Friday morning, London time, and urged the public to be "in no doubt that I can continue, thanks to the wizardry of modern technology, to communicate with all my top team to lead the national fightback against coronavirus".

Despite the best efforts of government spin doctors to downplay the consequences, Johnson's diagnosis is an extraordinary disruption of the smooth running of executive government during an unprecedented national emergency.