The source said a preliminary investigation was opened after two complaints were filed against Ramadan, a well-known figure in the Middle East.
He is the grandson of the founder of the Muslim Brotherhood movement in Egypt, Hassan al-Banna.
Ramadan took a leave of absence from Oxford last November after two women filed complaints in France alleging rape.
He has denied the allegations and filed a complaint for slander against author Henda Ayari, one of his accusers.
Tariq Ramadan: Islamic scholar in post-Weinstein abuse storm