Amanda Abbington has confirmed that she will star opposite her long-term partner Martin Freeman as Mary Morstan in the third series of Sherlock.

The 38-year-old actress who appeared as Miss Mardle in period drama Mr Selfridge revealed that her character, Mary Morstan, has "fantastic secrets" and comes between John Watson (Freeman) and Sherlock Holmes (Benedict Cumberbatch) as a "kind of third wheel".

Speaking to The Scotsman, Abbington said: "I remember doing a scene very early on with Martin and Ben, and I was standing in between them and I thought, 'I really have to up my game, this is proper.

"They really bounce off each other. When I was at the read-through, I remember watching them, and the chemistry between them is just amazing. They do have this beautiful relationship, so getting in between that - which is what Mary does, she becomes this kind of third wheel - was scary.

"Also, the fans love these two together, so I'm sure Mary won't be particularly liked by them, but I hope that on the whole people really like her, because she's a great character and she has some fantastic secrets."

Speaking about landing the part, Abbington added: "We were at Mark Gatiss's [co-creator] house when The Hounds of Baskerville first came on the telly. Afterwards they went into the kitchen and were sitting talking about the next series. I went in and sat with them.

"It had been mooted that I would be in it, might have a little part in it. They were talking about who might play Mary and I was like, 'So, who are you thinking of?' and they said 'You'.

"I got very emotional because it just seemed so amazing, just such a gift."