The actress, who plays the Time Lord’s sidekick Amy Pond, told Graham Norton she did not know if her character would be killed off.
She said: “I have known for ages that I am leaving and it’s such a massive relief that it is out and I can speak freely about it.
“I’m back next year for a few episodes but it’s not actually confirmed when I go.”
The actress, who appeared on The Graham Norton Show last night alongside actors Gerard Butler and Martin Freeman, and musician Noel Gallagher, said her latest role as 1960s model Jean Shrimpton got a vote of approval from the woman herself.
Gillan said: “She gave her consent to having it made but then didn’t want anything to do with it because she is quite reclusive.
“But she saw it and she really liked it and left a voicemail saying she thought it was really accurate.
“That was the best compliment I could have got.”