The pair, who have already begun filming, will appear on screen in the new year when George is unexpectedly reunited with his adoptive parents.
Eddie and Gloria arrive in Walford to attempt to rebuild bridges with their son George having not seen him for over a decade but it soon becomes clear that the pair are hiding something.
Counsell, who will be taking on the role of Gloria Knight, is best known for starring in the BBC television series Brush Strokes. Fairbank, who plays her on-screen husband, recently appeared in The Little Mermaid, Guardians of the Galaxy and Andor.
Speaking about the casting news, EastEnders boss Chris Clenshaw said: "We’re absolutely delighted to welcome the legendary and immensely talented actors, Christopher and Elizabeth, to the EastEnders family as they bring to life the characters of Gloria and Eddie.
"Although currently set to join for a short stint, Gloria and Eddie are thrust into the heart of the drama as they try to reconnect with their son but it soon becomes apparent why George has kept his distance."
Describing her delight at joining the show, Counsell said: "I can honestly say it has been an absolute joy. The cast and all of the team behind the scenes at EastEnders are brilliant and have been so welcoming. Colin and I actually worked together before, and who would have thought that I’d be playing his mum almost thirty years later!"
Fairbank added: "I can’t tell you how thrilled I am to join the cast of EastEnders. Everyone has been so welcoming, and it’s been fantastic to meet and work with the brilliant actors involved in the show."
EastEnders airs on Mondays - Thursdays at 7.30pm on BBC One. The show also streams on BBC iPlayer, where most episodes drop early at 6am ahead of their TV broadcast.
You Might Also Like