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

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 the usual rate of $135,000 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.

Viewers are expecting the likes of Anthony Edwards and Noah Wyle to slip back into the overalls of Chicago's County General Hospital for the 331st programme.

Julianna Margulies, who played Clooney's on-screen love interest, nurse Carol Hathaway, has also announced her imminent return to the show, which was once America's hottest drama and has won 21 Emmys.

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 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 the couple are happily married, and their nine-year-old twin girls are expected to appear.

"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 Clooney's return may feature in one of three episodes that precede the finale. In one of the other shows, Sarandon is scheduled to play a woman whose grandson is killed.