The new programme will be called Torchwood (an anagram of Doctor Who) and will follow a crack team investigating alien activities and crime in modern-day Britain.
It will feature in its starring role John Barrowman, who played Captain Jack Harkness in Doctor Who and who will play the same character in Torchwood.
Like the latest version of Doctor Who, which the BBC successfully relaunched this year, Torchwood will be based in Cardiff. Davies, who has just begun writing Torchwood, said the new programme would be aimed at adult audiences and would "have its own, unique identity". He said: "Torchwood will be a dark, clever, wild, sexy, British crime/sci-fi paranoid thriller cop show with a sense of humour - the X Files meets This Life," the latter a reference to the groundbreaking Nineties BBC drama about a group of young lawyers in Bristol.
Torchwood will be shown next summer on BBC3 in 13 episodes, each lasting 45 minutes. Alert viewers of the forthcoming Doctor Who Christmas special will hear a reference to the Torchwood unit and further mentions will be made in the new series in the spring.
Stuart Murphy, the controller of BBC3, described Davies as an "absolute genius" and described Torchwood as "a massive coup".
He said: "We had never done sci fi before and it is a genre which people treat in a certain way. You look at what he has done with Dr Who and we said to Russell what would you do with a post-watershed sci fi?"
Mr Murphy said he hoped that Davies would bring to the new series a similar quirky humour that the writer deployed in the period drama Casanova, which was also shown on BBC3 before moving to BBC1.
"Each episode of Torchwood will be a one-off story and will be funny in a way that Casanova brought humour to period drama," he said.
Captain Jack has been described as Doctor Who's "first openly gay companion" and a "hunky bisexual".
Davies told Doctor Who magazine earlier this year: "It wasn't me sort of dying to get a bisexual character on screen. Yes, I'm a gay writer, but I was thinking: Jack's from the 51st century so of course he's going to go out with men and women. To get hung up on it is almost too sad for words, frankly."
Torchwood will allow Davies to explore relationships a little further. Mr Murphy said of the new series: "The people have affairs with one another. There will be sex and swearing, I assume. I'm quite relaxed about that, it will be post-watershed and Russell can do it in a funny and sexy way."
Describing the idea behind the new show, the BBC controller said: "It's a renegade bunch of investigators who investigate real-life, normal crimes. They also look into alien happenings. They have been charged by the British government to find alien technology that has fallen to Earth and they need to do it without the FBI and UN knowing."
Mr Murphy said the new series would have a distinctly Welsh feel, reflecting Davies's Swansea roots. "It's set in modern-day Cardiff. Unlike Doctor Who, which made Cardiff look like Dickensian London, this will look like Cardiff."
Davies said he was especially pleased to have secured the services of the "fantastic" Barrowman and said that Torchwood "gives us the chance to further develop exceptional talent from Doctor Who".
Barrowman, who as Captain Jack is part of the Doctor's Tardis crew, was born in Glasgow but grew up in Illinois. He first came to prominence as a children's television presenter on the show Live and Kicking, where he worked alongside Emma Forbes and Andy Peters.
BBC3 hopes to begin screening Torchwood at the end of the next series of Doctor Who adventures, in which the Doctor will be played by David Tennant.
Torchwood is the latest morphing of one of the BBC's most famous shows, which first reached the screens in 1963, although the programme will be distinct from Doctor Who and no stories will directly cross over between the two projects.
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