James Murray, 41, son of the Earl of Mansfield, has been accused of raping a 16-year-old girl in his home after spiking her Ribena with vodka.
Oxford crown court heard that Murray befriended the girl when she was 15 and she was upset after failing her exams.
The two spent increasing amounts of time together, resulting in the girl repeatedly skipping school until a police officer met her to discuss the relationship.
"The police came round saying Jamie wasn't allowed to get in contact with me until I was 16," she told the jury.
However, she continued to see Murray, repeatedly visiting his home to watch his television and play computer games.
She told the court that she was at Murray's two-bedroom Oxford flat on a day in June when he attempted to give her alcohol. She refused and instead had a cola followed by a Ribena she believed was filled only with crushed ice.
But after the second drink she felt woozy, the court heard. She said: "I don't know if he put anything in it but after drinking the Ribena I suddenly felt really tired all of a sudden."
After going to sleep on an inflatable bed in the spare room, she said she woke up to find her tracksuit trousers pulled down below her knees, while Murray lay naked alongside her.
She told the court that he asked: "Do you remember anything about last night?" Then he told her: "I had sex with you when you were asleep."
Murray denies raping the girl, who is now 17. He claims that the two of them had a 10-day consensual relationship and slept together six times after she was 16.
The trial continues.