A former university student who put a young prostitute through a terrifying rape ordeal has been jailed indefinitely for public protection.

Mohammed Khan, who was studying at Greenwich University on the Chatham Campus, will stay behind bars for almost six years before the parole board will consider his release.

Judge Michael Lawson, QC, said had he imposed a determinate sentence, it would have been of 13-and-a-half years.

The victim said afterwards: “It’s not long enough – he should have got life. He will be in prison and can forget about it. I have to live with it for the rest of my life.”

Khan, 21, formerly of Ordnance Street, Chatham, was convicted of three rape charges in November.

He told his victim to shut up, that he had murdered 20 people that evening and would make her the 21st if she didn’t.

The court heard how Khan trawled websites for call girls and escort agencies.

His victim was on her way to meet a customer in a Rochester park on April 1 last year, when he grabbed her, dragged her towards tennis courts and put his hand over her mouth to silence her. She screamed when she saw others in the park, but was ignored. She recognised her attacker as a punter who had paid her with two dud £20 notes about a month earlier.

Khan repeatedly raped her, then forced her to perform a sex act on him, attempting to ensure no DNA evidence was found.

After her ordeal she contacted a friend and reported the rape to the police. The victim later picked Khan out of an identification procedure.