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If Boris Johnson dies or is incapacitated, who steps in? In the short term at least, that would be his foreign secretary, Dominic Raab, who was deputized by the British prime minister before he was taken to the hospital suffering from COVID-19.

But somewhat surprisingly for one of the world’s oldest parliamentary systems of government, prime ministerial succession is not entirely clear in Britain, whose constitution is unwritten.

Johnson, 55, was moved Monday to the intensive-care unit at a London hospital, his office said. Raab, 46, was named, before the prime minister’s condition worsened, to handle government business “where necessary,” but the longer term — if the need arose — would probably prompt some questions.

Johnson is a head of government, not a head of state. That would be Queen Elizabeth II, and the royal line of succession is well-enshrined: The throne would pass first to her eldest son, Prince Charles, 71, then Charles’ eldest son, William, 37, and then William’s three young children, in order of birth.

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