Myles first featured in an episode of 'Doctor Who' last year as Gwen, the character she reprises for 'Torchwood'. Previous roles include 'Soundproof' for BBC Two and 'Belonging' for BBC Wales.
Davies, who previously wrote 'Queer as Folk' and the hugely successful resurrection of 'Doctor Who', said: "I've admired Eve's work for years, and when she was able to guest star in 'Doctor Who' last year, it just confirmed to me that she was one of Wales's best-kept secrets.
"The part of Gwen in 'Torchwood' was written specially for her, so it's a good thing she said yes, Eve and John Barrowman have already met up, and the combination is going to be electrifying."
Myles will will star alongside former 'Live and Kicking' presenter Barrowman, who returns in the role of Captain Jack.
The 13-part drama series is aimed at a post-watershed, adult audience and has been described as a mix between 'X Files' and the mid-1990s drama series 'This Life'.
'Torchwood', an anagram of 'Doctor Who', is due to start filming this summer and will debut on BBC Three this autumn. It will air at a later date on BBC One.
The series is being produced by BBC Wales in Cardiff, as was the last 'Doctor Who', but the storylines from the shows will not cross over.
The second series of 'Doctor Who' is due to air later this year, with 'Casanova' star David Tennant as the Doctor and Billie Piper as his assistant Rose.
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