Johnson is recovering at his country residence after he was hospitalized with COVID-19. His foreign minister, Dominic Raab, is standing in for him while he recovers.
"Yesterday he sent a message of condolence to (Canadian Prime Minister) Justin Trudeau after the very sad loss of life in the shooting in Canada. Later today ... he will be speaking to President Trump," the spokesman told reporters.
Later this week, the prime minister is also expected to have an audience with Queen Elizabeth, the spokesman said, adding that it would be the first such meeting for three weeks.