George Clooney’s return to ER sparks huge bidding war

Advertising slots for the final two hour episode have tripled as star becomes a married father of twins

He achieved fame as Hollywood's perennial bachelor, but George Clooney has sparked a multimillion-dollar bidding war by agreeing to return to the television series ER in the role of a married middle-aged father of twins.

The broadcaster, NBC, announced last week that it was tripling the price of 30-second advertising slots during the show's last-ever episode, a two-hour special, from their usual rate of $135,000 (£95,000) to between $425,000 and $500,000. Its decision came amid growing public excitement about the remaining four episodes of the 15-year-old medical drama, which will feature guest appearances from Clooney and Susan Sarandon, along with a string of prominent ER alumni.

NBC's decision to pull the ER franchise – which was created by the late Michael Crichton and is now controlled by John Wells, one of the most prominent TV producers in Hollywood – highlights a worrying trend for television drama, which is being replaced on the old networks by more profitable reality programming. ER has some of the highest production values in US television, costing $12m an episode to film. Reality programmes cost a fraction of that amount.

But while ER's audiences have steadily fallen from their mid-1990s peak of 27 million – according to Nielsen, the market research company, Thursday's episode pulled little more than seven million viewers – reality shows can still secure mass audiences in an increasingly fragmented market. The biggest weekly show on US television is American Idol, which is watched by more than 26 million people on the two nights a week on which it is broadcast.

ER was once America's hottest drama and has won 21 Emmys. NBC hopes to strike back with its 331st episode and last hurrah, which is due to air on 2 April. Viewers are expecting the likes of Anthony Edwards and Noah Wyle to slip back into the overalls of Chicago's County General Hospital and Julianna Margulies, who played Clooney's on-screen love interest, nurse Carol Hathaway, has also announced her imminent return to the show.

Plot details remain a closely guarded secret. However, Margulies revealed last week that she and Clooney had spent two days filming at Warner Studios in Burbank, Los Angeles, where the show's off-location scenes are shot.

The appearance of Hathaway with Clooney's alter ego, Dr Doug Ross, will provide a "happily-ever-after" footnote to their off-on romance which, despite its status as one of the enduring love stories in modern TV drama, was never properly concluded. Fans will learn that the couple are now happily married to each other, and are about to see their nine-year-old twin girls.

"You pick up a slice of their life now. It's very satisfying," revealed Margulies in an interview with TV Guide, adding that although loose ends will be tied up, the couple's history in the years since the womanising Ross left "could be a whole other show". During filming, she added: "It felt as though a lifetime had happened since [they left the show in 2000] ... But nothing had happened since then ... Half the crew is still there, God bless them. We were all family. It was like coming home."

Sources at Warner Brothers, who were not authorised to speak publicly, said that Clooney's return may feature in one of the three episodes that precede the finale, which airs in the UK some time in June. In one of the other remaining shows, Sarandon is scheduled to play a woman whose young grandson is killed.