Johnson was in intensive care on Tuesday after receiving oxygen support for serious COVID-19 complications, leaving Raab to lead the government's response to the accelerating outbreak.
Johnson's personal battle with the virus has shaken the British government just as the United Kingdom enters what scientists say is likely to be one of the most deadly weeks of the pandemic, which has killed 5,373 people in Britain and 70,000 worldwide.
Johnson, 55, was admitted to St Thomas' Hospital across the River Thames from the House of Commons late on Sunday after suffering persistent coronavirus symptoms, including a high temperature and a cough, for more than 10 days.
His condition rapidly deteriorated over the next 24 hours, and he was moved to an intensive care unit, where the most serious cases are treated.
Although he had received oxygen, his office said on Monday he was still conscious and was moved to intensive care in case he needed to be put on a ventilator.
Johnson is the first leader of a major power to be hospitalised for the new coronavirus.