LONDON — Britain’s defense secretary, Michael Fallon, has resigned over allegations about his past conduct, amid a growing outcry over claims of sexual harassment or inappropriate behavior by British politicians.

Mr. Fallon’s departure from his ministerial post was announced by the office of Prime Minister Theresa May, who released a formal letter of resignation on Wednesday night.

“A number of allegations have surfaced about MPs in recent days, including some about my previous conduct,” wrote Mr. Fallon, referring to members of Parliament.

“Many of these have been false but I accept in the past I have fallen below the high standards that we require of the Armed Forces that I have the honor to represent,” he said, adding that he had “reflected on my position and I am therefore resigning as Defense Secretary.”