FILE PHOTO: Author Tariq Ramadan talks with a journalist after a conference at the Er-Rahma mosque in Nantes, France, April 25, 2010. REUTERS/Stephane Mahe/File Photo

PARIS (Reuters) - Swiss academic Tariq Ramadan, a professor of Islamic studies, was ordered to remain in custody in France on Friday and notified that he was the target of a full investigation into rape accusations, a source with the prosecutor’s office said.

Ramadan, who denies the allegations, took temporary leave in November from his post at Britain’s University of Oxford.

He vowed to sue for slander two women whose rape accusations prompted a preliminary judicial inquiry.

Friday’s announcement turned that preliminary inquiry into a full, open-ended investigation where French magistrates stated goal is to establish whether Ramadan should stand trial.

In France, the launch of a full investigation implies that initial findings justify further pursuit in the view of the investigating magistrates but does not mean the matter is sure to go to trial.

Ramadan is a grandson of Hasan al-Banna, an Islamist thinker and activist who founded the Muslim Brotherhood. He enjoys a substantial following among young Muslims and has challenged French restrictions on wearing veils.

Police arrested him and took him into custody earlier this week in France.