In a letter sent to Prime Minister Theresa May on Wednesday, Fallon said his conduct has “fallen below the high standards” required of the country’s armed forces, Aljazeera said.
“I have reflected on my position, and I am therefore resigning as defence secretary,” Fallon said in the letter.
Fallon was accused of repeatedly touching a female journalist’s knee during a dinner in 2002.
May accepted Fallon’s resignation and praised him for his role in the fight against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) in Iraq and Syria.
The prime minister has called a meeting to discuss the growing controversy over sexual harassment.
Meanwhile, Prime Minister Theresa May has appointed a new Defense Secretary, just as a growing sexual harassment scandal added to the pressure on a government already struggling with the fallout from Brexit.
May named a key Conservative Party fixer, Gavin Williamson, to fill the post vacated after the resignation of Michael Fallon, who admitted his behavior towards women had “fallen short” during his career.
CNN said Fallon was the first minister to resign after a swirl of allegations swept British political circles in recent weeks.
Williamson, 41, has been the government’s Chief Whip since 2016, in which role he was responsible for enforcing parliamentary discipline within the party.
His rapid elevation to a top Cabinet position has prompted questions in some quarters over his qualifications and whether, in effect he promoted himself, since the chief whip advises the Prime Minister on appointments.