Olivia Colman has told how her Broadchurch co-star David Tennant was livid when he discovered she was privy to the identity of the show's killer - when he had been kept in the dark.

The closely-guarded secret had kept the nation guessing when the series first aired two years ago and former Doctor Who stars Tennant was under the impression that the entire cast was unaware who did it while they filmed.

But as the pair appeared as guests on BBC One's Graham Norton Show tonight they explained that Colman had been in on the ID from the very start, and had even had to play along with discussions despite knowing the truth.

She admitted: "I knew. I asked at my first audition and they told me - and then they called me an hour later and said, 'we're not going to do it that way, nobody is going to know, so you're not allowed to know'. So I kept the secret."

Tennant said: "About halfway through series one I was talking to my agent about something and she said, 'So no one knows?' and I said, 'No, absolutely nobody knows' and she said, 'So it's just Olivia that knows?'

"I was like 'Olivia doesn't know, we've spent weeks together, we've discussed who it might be and we've taken bets on who the killer is'. She kept that secret brilliantly for months."

Colman added: "He was so angry when he found out I knew."

The couple are currently starring in the second series of the show, which is set on the Dorset coast and said it had become more difficult to keep the show's twists under wraps because of the increased public scrutiny as they film scenes.

"You are very aware when you are out doing a scene on the beach or at the end of the pier that there are people licking ice creams two feet away from you. I think, 'I can't say this loudly or it's all going to be given away'."