A decision has been made to move Bristol's homegrown hit TV series Casualty to Wales, the Bristol Evening Post understands.

Jana Bennett, the head of BBC Vision, is expected to make the announcement during a keynote speech to the Royal Television Society (RTS) in London tonight.

Her speech, titled "BBC Beyond the M25", will, according to the RTS, "unveil how the BBC plans to significantly grow output and commissioning in the nations and achieve half of all network output outside London by 2016."

Any such announcement is certain to be greeted with dismay this side of the River Severn.

Casualty's production here is estimated to be worth £10 million annually to the local economy.

The series has been made in Bristol for more than 20 years and Bristolians have become used to seeing Casualty production crews and stars filming around the city..

A series of warehouses in St Philip's house the sets which millions of viewers know as the inside of fictitious Holby Hospital's accident and emergency unit.

Over the years, Bristol and the fictitious city of Holby have become interchangeable, with sights such as the Floating Harbour and the Clifton Suspension Bridge making regular appearances in the drama.

Many believe the show will now end up being made in Cardiff, best known as the production centre for Doctor Who and Torchwood.

The aim would be to create a specialist BBC drama centre in the Welsh capital.

Such a move, rumours of which first surfaced some months ago, would be controversial.

Until Casualty came here, Bristol had been devoid of a BBC drama department since the Sixties. In its wake have come other shows such as Teachers, Skins and Mistresses.

Bristol is also home to the world-famous Natural History Unit and popular factual shows such as Antiques Roadshow and DIY SOS.

Whatever happens, the Casualty set, in common with those of all major TV drama series, will soon have to be rebuilt because the arrival of high-definition television means flaws are more visible.

Casualty, the longest-running emergency medical drama series in the world, was born in 1986. Created by Jeremy Brock and Paul Unwin, it brought something entirely new to BBC One's Saturday night schedule.

Years before the arrival of American counterparts such as ER and Chicago Hope, the emergency room at Holby – which was inspired by the work of the A&E department at Bristol Royal Infirmary – soon had viewers fixed to their screens.

With a cast that included classical actors such as Brenda Fricker and Derek Thompson, Casualty was an instant hit.

Derek, the only remaining original cast member, recently told the Bristol Evening Post: "It certainly doesn't feel like more than 20 years. I've worked on some week-long jobs that have felt like a life sentence, but I've always enjoyed working on Casualty because there's a constant stream of guest actors, so each episode feels different."

Over the years, patients have been played by famous names including Norman Wisdom, Eric Sykes, Phill Jupitus, Michael Jayston, Michele Dotrice, Ronni Ancona, Emma Samms, Lee Ryan, Nikki Sanderson and Johnny Briggs.

Bristol has now become home to Belfast-born Derek, who plays Charlie Fairhead. Speaking to the Evening Post's Seven magazine last year, he said: "I took to the place straight away. The people in Bristol had a similar manner [to people in Belfast] – they seemed friendly and approachable.

"That's why I've carried on with Casualty for as long as I have. I wouldn't want to leave a show that gives me the chance to live here."

For the first six years, most external shots of the hospital were filmed at the City of Bristol College – until a new exterior set was built in St Philip's in 2002.

In 2001, the show's popularity resulted in a switch from a traditional seasonal format to an almost year-round transmission.

The series has produced a number of spin-off programmes, including Holby City, Holby Blue and Casualty 1907. But Casualty remains Holby's core export.

In 2007, it won the BAFTA for best continuing drama.

Over the years, the folk of Holby have suffered just about every calamity you could think of, from an IRA bomb blast to a fairground ride disaster.

Scriptwriters have included the likes of Barbara Machin, of Silent Witness and Waking The Dead fame, who started her career on the series and returned to act as a script advisor.