Media playback is unsupported on your device Media caption Writer Russell T Davies and the show's stars explain the concept behind the fourth series of Torchwood

The creator of Torchwood has promised that the sci-fi show will stick to its Welsh roots despite its move to America.

Russell T Davies was speaking at the launch of Torchwood: Miracle Day at the BFI in London on Monday.

"I would never do a Torchwood that didn't have any links to Wales," the writer told the BBC News website.

The new 10-part series imagines a world where suddenly nobody dies.

The show sees the return of John Barrowman as Captain Jack Harkness and Eve Myles as Gwen Cooper.

New cast members include American actors Bill Pullman, as a convicted child murderer, and Mekhi Phifer, as a CIA agent.

Much of the action takes place in the US, with scenes in Wales and other locations around the world.

The series will air in the US in early July and be shown later in the summer on BBC One.

Image caption John Barrowman's Jack Harkness first appeared in Doctor Who in 2005

For the first time, BBC Wales is making Torchwood in partnership with US entertainment network Starz, along with BBC Worldwide.

"In a time when budgets are tight, it's great to have an infusion of cash from America that we can have fun with - and do some big brave stuff with," said Davies.

Executive producer Julie Gardner said: "We wanted to do a global threat with more than one location and the co-production enables that to be possible.

"The joy of it wasn't that we sold the format to the Americans and then waved goodbye to it.

"Of course budgets are shrinking, but it was about working smart. In pooling the money we got a bigger show for a better price."

Welsh links

Torchwood star John Barrowman stressed that the American location had not changed the show - which is normally centred on Cardiff.

"Loyal fans will recognise it in the first second," he said.

This is the fourth series of Torchwood, a Doctor Who spin-off which launched on BBC Three in 2006.

The character of Captain Jack had first appeared in Doctor Who the previous year.

Torchwood later moved to BBC Two, and then to BBC One.

Davies said his storyline about nobody being able to die had its roots in medieval myths.

Image caption Eve Myles (right) returns as Gwen along with Kai Owen as her husband Rhys

"I wanted to take that and apply it to the real world," he said. "You realise how much the economy and the healthcare system is build around mortality. When people stop dying, very soon it starts to collapse."

Davies, who is now based in Los Angeles, pointed out that his story never left Wales for very long.

"It starts in Wales, they go on the run across the United States, but they come back to Wales. I don't want people to think we've abandoned the homeland."

The first episode, screened to the press and public at Monday's launch, included a joke about the Severn Bridge toll, as well as a helicopter action sequence on Rhossili beach on the Gower coast.