Steven Moffat claims Russell T. Davies has turned down offers to return to 'Doctor Who'.

The sci-fi franchise's showrunner has extended several invitations to the screenwriter to pen an episode for the show, but Steven believes Russell wants a "nice long rest" from being involved with the popular BBC drama series from 2005 to 2010.

Steven told Digital Spy: "I do keep asking him [to write a new episode]. The offer is continually made and I'm getting nowhere!"

Although Russell has not expressed interest in returning, Steven would welcome his friend back "like a shot" as he believes the Welsh producer is "the best writer breathing".

He added: "I think he probably did his duty ... for 'Doctor Who' so he's maybe wanting a nice long rest from it."

The show is currently planning its 50th anniversary episode - which will take place in November - and Russell previously stated he doesn't want to know what the storyline will involve as he wants to enjoy the experience as a fan, not a member of the production team.

He said: "I have asked current boss Steven Moffat not to tell me what they are planning."

Meanwhile, Nick Hurran confirmed last week he is set to direct the milestone birthday celebration episode after his success with long-running BBC One sci-fi show 'Asylum Of The Daleks' in addition to the dramatic mid-season finale, 'The Angels Take Manhattan'.

'Doctor Who' will return to TV screens on March 30.