The special episode, centred around the Mitchell family, will air this autumn.
Jaime Winstone played Dame Barbara Windsor in BBC One's 2017 drama Babs and will lead the cast as Peggy Mitchell as viewers are transported to the winter of 1979.
The Mitchells are hit hard by a Britain in economic turmoil and Peggy has her hands full, between keeping the boys, Sam and the household in line whilst her marriage to Eric is showing its cracks. And tension rises between Phil and Eric as they both vie for the title of man of the house.
Elsewhere, Archie is up to his usual tricks as Glenda does her best to keep the lively Ronnie and Roxy out of trouble.
As present day Phil grapples with growing pressure from DCI Keeble, EastEnders fans will take a journey back to his youth where clues to his present will undoubtedly be revealed.
This unravelling of the Mitchells’ chequered past marks the beginning of a tumultuous journey for Phil as he is forced to confront some long forgotten truths. How will revelations about the past influence Phil’s ability to protect his family in 2022?
The episodes will also feature characters from EastEnders history that have never been seen on screen before such as Peggy’s husband Eric, played by George Rosso, Billy Mitchell’s older brother Charlie, played by Charlie Heptinstall and their father Stephen, played by Dean Roberts.
Speaking about taking on the role of a young Peggy Mitchell, Jaime Winstone said "It’s been such an honour to step into Peggy Mitchell’s shoes for the first time, and into Barbara’s shoes for the second time."
"There’s certainly a healthy amount of pressure when you’re taking on such an iconic role - I thoroughly enjoyed it. Playing this role has really cemented my love and admiration for Barbara as I got to bookend my journey by playing Peggy Mitchell – I hope I’ve done her justice and I’ve loved every minute of it, I can’t wait for everyone to see the episode."
To which EastEnders' Executive Producer Chris Clenshaw added "We’re delighted to be welcoming an incredible cast of actors to set for this special flashback episode, including the luminous Jaime Winstone as a young Peggy Mitchell."
"Focussed on the Mitchell family, audiences will embark on a rollercoaster journey to the East End of London in 1979 where they’ll delve into the family’s past, seeing much-loved and iconic characters, as well as meeting characters from the show’s history that have never been seen on screen before. There are twists, turns and secrets revealed – after all, trouble is never too far away where The Mitchells are concerned!"
Also joining the cast are Daniel Delaney as Phil, Teddy Jay as Grant, Henry Garrett as Archie, Rose Reynolds as Glenda and George Greenland as Billy.
EastEnders continues Mondays to Thursdays at 7.30pm on BBC One