The victory may be propelled by what is seen as a wave of sympathy votes in a country still reeling from the shock of the former prime minister's assassination.
His resignation will trigger an internal election to pick a new leader of the Conservative Party, who will also be the next prime minister. He steps down after defections left him unable to govern.
Johnson was booed during the queen's Platinum Jubilee — the latest sign of intense public anger over his flouting of his own government's COVID-19 rules.
Australians awoke on Sunday to a new prime minister in Anthony Albanese, the center-left Labor Party leader, though it remains unclear whether his party can form a majority government.
Sri Lankan Prime Minister Mahinda Rajapaksa has tendered his resignation amid an economic crisis and violent protests against he and his brother, the country's president.
British police said Tuesday that they were fining 20 people over parties held by Prime Minister Boris Johnson and his staff during coronavirus lockdowns.