Police revealed on Monday they are hunting a suspected serial rapist they believe has carried out five sexual assaults since last October. Investigators decided to go public after rumours a serial attacker was on the loose had spread through social media.

Concerned university chiefs have now taken the step of writing to their 30,000 students warning them not to go out alone at night.

“We ask you to be careful when you are going from one place to another. Please do not go out unaccompanied and stay vigilant. This especially concerns female students,” wrote Jacques Comby, President of the University Lyon III.

Since October last year the attacker is believed to have struck at least five times in the eighth arrondissement of the city at around midnight.

In each case he has threatened his victims with a knife, leaving some of them with cuts to the skin, according to reports in French media. Police sources told French daily 20 Minutes that the attacker always wears a hood to help hide his identity.

His victims are all believed to be in their twenties. The last reported attack took place at the end of January.