Eliza Dushku, who has already appeared in the show as Faith, will take over from Gellar who is leaving to pursue her film career, according to the Express newspaper.
Dushku will reportedly keep the role of moody Faith, rather than becoming the new Buffy.
The move would leave the door open for Gellar to come back and make the occasional cameo appearance.
It has not yet been announced when Gellar will leave the series.
Before taking over, Dushku is set to to appear alongside Gellar in the final three episodes of the current series, which is shown on Sky One on Thursdays.
The show's producers had looked at several different options before settling on 22-year-old Dushku, said the paper.
Other ideas considered included making Buffy's younger sister Dawn the main star.
Gellar decided to leave the show last summer to concentrate on her film career.
She has already starred in the live action film Scooby-Doo, alongside husband Freddie Prinze Jr, and Cruel Intentions.
Gellar made her name with Buffy, which has become one of the most popular teenage TV shows worldwide.
She built up a cult following as a high school student with supernatural powers to hunt down and dispatch vampires.
Dushku is not nearly as well known. She was discovered at the end of a five-month search throughout the United State to play the lead role of Alice opposite Juliette Lewis in the film That Night.
She has appeared in several films and has worked alongside the likes of Leonardo DiCaprio, Robert DeNiro and Ellen Barkin.
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