"I have written to my association in Uxbridge and South Ruislip to say that I am stepping down forthwith and triggering an immediate by-election," Johnson was quoted as saying by UK broadcaster.
Johnson told reporters that he decided to stand down after having received a letter from an independent committee investigating whether he deliberately mislead the parliament on the parties at his office during COVID-19 lockdown.
"I am very sorry to leave my wonderful constituency... But above all, I am bewildered an appalled that I could be forced out anti-democratically," he was quoted by the British media as saying.
Johnson was eventually forced to resign as UK Prime Minister last July after a series of leaks showed that several social gatherings had been held at his offices throughout 2020 and 2021, flouting COVID-19 social distancing rules. The situation was aggravated by the reports that two more parties were held in April 2021, right before Prince Philip's funeral, when the UK still maintained COVID-19 restrictions and was in national mourning. Johnson later apologized and said he had paid the fine issued by the Metropolitan Police.
In March, Johnson admitted he had misled the parliament on the matter of parties held at his residence at 10 Downing Street during the nationwide lockdown, however argued he did it "in good faith."