In an exit described as the "most moving" ever filmed, Pat will discover she has untreatable cancer after collapsing in her kitchen.
Doctors send her home, where Pat's closest family and friends rally round to support her during the final days. Michael French will return as Pat's son David Wicks, the Daily Star Sunday reports.
EastEnders cast and crew were said to be in tears as Pam St Clement, who plays Pat, filmed her final scenes on Tuesday. She announced her departure back in July.
"Pat's exit will be epic," a source said. "Many of her family, including stepson Ricky Butcher, his wife and Pat's granddaughter Bianca can't handle the news. Even twisted stepdaughter Janine Malloy struggles to cope.
"But in true Pat style she puts on a brave face and deals with what's happening to her with dignity. Viewers will see her pour one last gin and tonic from the bar in her front room before she fondly looks at photographs of her extended family."
Viewers will apparently see Pat make an emotional toast to husband Frank Butcher shortly before her death.
"She takes one last look around the house she's spent so many years in before going to her bedroom. Pat isn't afraid of dying as she knows she'll be joining Frank. She's courageous and tough right up until the end. But as she's coming to terms with having cancer her heart gives up."