Downing Street has drawn up a “designated survivor” plan to ensure the continuity of the government if Boris Johnson or his top aides fall sick, after a host of senior officials were forced to self-isolate last week. Dominic Raab, the foreign secretary who is also first secretary of state, will stand in for the prime minister if he contracts the coronavirus — but the decision has unleashed acidic exchanges among ministers jostling to be next in line.

Johnson has no virus symptoms but has not been tested. Raab has twice tested negative after contracting a cough.

The “designated survivor” plan, which echoes the plot of a television series in which Kiefer Sutherland’s character becomes president after the rest of the cabinet is killed in a