Image copyright BBC / Getty Image caption British comedians Lucas and Davies are surprise additions to the Christmas episode

Little Britain's Matt Lucas and The Inbetweeners' star Greg Davies are joining the cast of Doctor Who for this year's Christmas special.

Producers have confirmed Lucas, who starred in Pompidou and Bridesmaids, will play a character called Nardole.

Cuckoo star Davies will play King Hydroflax in next month's episode, the first time 12th Doctor Peter Capaldi will meet Alex Kingston's River Song.

A new synopsis has revealed the Doctor will be recruited into her squad.

"It's Christmas Day on a remote human colony and the Doctor is hiding from Christmas Carols and Comedy Antlers," reads the synopsis, released by the BBC along with a first-look picture from the episode.

"But when a crashed spaceship calls upon the Doctor for help, he finds himself recruited into River Song's squad and hurled into a fast and frantic chase across the galaxy."

Image caption The first image released from the Christmas special shows Alex Kingston as River Song, with the Doctor played by Peter Capaldi

It also revealed what might be in store for the characters played by British comedians Lucas and Davies.

"King Hydroflax is furious, and his giant Robot bodyguard is out-of-control and coming for them all! Will Nardole survive? And when will River Song work out who the Doctor is?

"All will be revealed on a starliner full of galactic super-villains and a destination the Doctor has been avoiding for a very long time."

'Perfect couple'

Doctor Who boss Steven Moffat told the Radio Times the Christmas episode would be a "great romp", but has warned fans there is "not a lot of Christmas in it".

"The big deal is Doctor number 12 encountering River and vice versa," said Moffat. "It's strange because they're now, in our human terms, the perfect couple. They're both sexy older people.

"It's always been slightly strange before with Matt Smith. Now they could actually be married. It works."

Moffat said River provided a nice bridge for fans mourning Clara Oswald, played by Jenna Coleman, who was killed off last weekend.

"We've just lost Clara, so I didn't want to go straight into a new companion," said Moffat.

"I'll be honest, I brought River Song back in because I thought there was a possibility I'd never write [Doctor Who] again, so that'd be my goodbye.

"But also, I really fancied it. I hadn't written River for a couple of years, and I'd missed her."

Image caption Dickensian will feature some of Charles Dickens' most-loved characters

Doctor Who is just one of the highlights of the BBC's 2015 Christmas schedule announced on Tuesday, along with the return of Luther and Sherlock.

There will be Christmas specials of EastEnders, Top of the Pops, Strictly Come Dancing, The Detectorists and Call the Midwife - in which Nonnatus House will be "rocked" when Sister Monica Joan goes missing.

Dickensian sees Charles Dickens's most famous characters - including Scrooge, Fagin and Miss Havisham - intertwine in 19th Century London, and there is also an adaptation of the best-selling crime novel of all time, Agatha Christie's And Then There Were None.

Image caption Agatha Christie's And Then There Were None features a host of acting talent including Poldark's Aidan Turner (far left)

Image caption Bruce Forsyth will return to Strictly Come Dancing to host the Christmas special with Tess Daly

Younger viewers will see the return of Harry Hill as Professor Branestawm, a "magical" adaptation of Julia Donaldson's Stickman voiced by Martin Freeman, Hugh Bonneville and narrated by Jennifer Saunders; and BBC One's first Shaun the Sheep 30-minute special, The Farmer's Llamas.

David Walliams will star in an adaptation of his children's book Billionaire Boy, as well as hosting a brand new sketch show with special guest Joanna Lumley, while Catherine Tate returns for two new episodes of Nan.

Other highlights include a new series from David Attenborough uncovering the history and secrets of the Great Barrier Reef and Charlie Brooker's 2015 Wipe review of the year.