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Doctor Who producers are bringing back flirty female favourite River Song to serve up a Christmas Day “sex storm” for fans.

They decided to include River - played by Alex Kingston - as surprise guest star in the raunchiest-ever episode of BBC 1’s long-running sci-fi show.

However, despite her shock festive return, Doctor Who , played by Peter Capaldi , still faces being lonely this Christmas because River doesn’t recognise his new reincarnation and keeps romancing other fellas.

The lovelorn Timelord ends up chasing River around the universe in a vain bid to to win over her affections.

Show-runner Steven Moffat admitted that the Christmas special wasn’t very festive - but there’d be “lots of kissing” and flirting as Doctor Who turns into a saucy rom-com.

(Image: BBC)

He revealed: “It is not the most Christmassy Doctor Who ever… but it is a funny, fast madcap chase episode - with the doctor being chased and also chasing River.”

He told how he decided to bring back River, who was last seen on screen in 2013, as the perfect female foil for Peter Capaldi after speaking to his predecessor Russell T Davies.

READ MORE:Jenna Coleman says Doctor Who should be returned to traditional teatime slot

Moffat revealed: “Russell said ‘you’ve got to bring her back - she has got to be together with Capaldi - it will be a sex storm.’

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“So I have - though the episode is called ‘Sex Storm’ - i couldn’t get that past the controller - but it is a wonderful episode.

"There is wonderful chemistry between Peter and Alex. It had to be done.’’

BBC insiders said the decision to bring back Alex, 52, was a reaction to critics who claimed Peter was tooled to be travelling with Clara Oswald, played by Jenna Coleman, 29.

Peter said: “It is a lighthearted fun thing River doesn’t recognise the Doctor and he gets upset because she is flirting and doing all sort of stuff with people.

(Image: Getty)

“She simply cannot be persuaded that he is the former Matt Smith/David Tennant/Christopher Eccleston - so there is lots of kissing, but not between River and the doctor!

“It was great working with Alex. She is an old hand on the show - it was brilliant fun.”

Moffat added: “The Tardis is covered in Icicles at the beginning but it moves on quite fast from that … basically you become aware of how many times you have done Christmas specials .

“Eventually you run out of things to do - or you end up with “deadly snake tinsel”

River first appeared in 2008 and has so far featured in 15 episodes as Moffat described a “naughty less moral” version of the Doctor.