The Knock You Down hitmaker checked into The May Fair Hotel, but didn't realise her room had been doubled-booked - and she had no idea Lavigne was in the suite when she entered with her 10-strong entourage.
Hilson reveals her security guard went in first and Lavigne was given a serious fright when she spotted an unknown man in her room.
Hilson tells Britain's Daily Mirror, "My bodyguard went into the bedroom and saw a girl sitting on a chair who almost jumped out of her skin. Her magazines were all laid out on the bed. I thought, 'Wow, this is a good service if it comes with the hotel!'.
"When I was told it was actually Avril Lavigne's room, I said, 'Oh, my God!'"
As reported by ContactMusic.com