Rex Features
The 72-year-old - who played Detective Inspector Maggie Forbes in the '80s TV shows - first appears in Walford as Glenda in January, much to the shock of her daughters Ronnie (Samantha Womack) and Roxy (Rita Simons).
Despite having never been seen on screen, Glenda's potted backstory has formed through her children, ex-husband Archie (Larry Lamb) and sister-in-law Peggy (Barbara Windsor).
Described as Archie's "first beautiful, quiet and dutiful wife", Glenda walked out on him and the girls around the time Ronnie gave birth at the age of 14. Archie's controlling nature turned the once brash and confident Glenda into a nervous wreck and when Archie orchestrated their teen daughter's child Amy to be taken away after the birth, Glenda fled to Australia to start a new life.
Their mother's sudden departure from their lives left Ronnie and Roxy wondering exactly why she abandoned them and while Roxy resolved never to forgive her, Ronnie can understand why she made a break for freedom.
However, Glenda's sudden arrival on the Square stuns the Mitchells as Archie's meek and mild first wife has apparently evolved into a "polished, demure, dynamic and assured" woman.
Her shock appearance is rumoured to tie in with this year's Christmas storyline twist involving Archie.
Of her new role, Gascoine said: "I'm thrilled to be joining the cast EastEnders and I am really excited to be back on British TV."
Executive producer Diederick Santer commented: "I'm so looking forward to welcoming the hugely talented Jill to EastEnders. I have a very clear memory of watching C.A.T.S. Eyes every Friday night, while - to the fury of my mother - munching my way through an entire packet of Jaffa Cakes."
He added: "Jill's character Maggie Forbes was always my favourite - tough, glamorous, and exciting - perfect qualities for Glenda Mitchell."
> 'EastEnders' Christmas plot revealed
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