Gillian's reappearance will tie in with Lacey Turner's permanent return to EastEnders as Stacey Branning later on this year. The two were last seen on screen together on Christmas Day in 2010.
"I feel very honoured to be asked to reprise my role as Jean Slater, for a short while," said Gillian.
"The storyline promises to be challenging and exciting! I look forward to playing again with old friends and new and to pick up the special relationship between Stacey and Jean."
Our Girl actress Lacey announced her return to the soap last December. Viewers of the show would have seen Stacey make a return to Albert Square in February shortly before disappearing from our screens again after confessing to the murder of Archie Mitchell. Stacey is currently awaiting trial in prison.
Gillian is best known for her portrayal of Jean's struggle with bi-polar disorder. Other notable storylines include Michael manipulating Jean out of her lottery winnings and her estranged relationship with her son Sean, as well as her friendship with Shirley.
Actress Gillian, who announced her departure from the soap last July, made her exit from the show in September 2013 when she left Walford to start a new life with her boyfriend Ollie.
Executive producer Dominic Treadwell-Collins added: "We are so pleased to have Gillian back with us to reprise her role as Jean Slater, albeit briefly. Stacey and Jean have such a rich, loving but difficult history.
"There is still so much to plumb between them as we delve deeper into what it means to have bi-polar disorder as a mother but also as a daughter. There are some truly heartbreaking scenes ahead for our audience."
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