Johnson, 55, will take back control of a government under pressure from the economic fallout of shutdowns aimed at curbing the spread of the highly infectious virus, as well as a rising death toll.
As of Saturday, Britain has recorded more than 20,000 deaths from COVID-19, the disease caused by the virus.
Criticism is growing over the government response to the pandemic, with limited testing and shortages of protective equipment for medical workers and carers.
Johnson's stand-in leader Dominic Raab has faced questions over how Britain will ease the lockdown without a deadly second wave of infections.