Back in the summer, Cora claimed that Ava died shortly after being born, but next week's episodes feature the big revelation that she was lying.
As first reported earlier today (November 6), Cora confides in Patrick Trueman by revealing that Ava is still alive.
Mitchell told Soaplife of the storyline: "The baby's father was black and he left Cora before he knew she was pregnant. She was 18 and unmarried, and at that time, having a baby in her situation was a social stigma. She was made to feel enormous shame by her parents, who forced her to give the baby up for adoption."
EastEnders bosses have already confirmed that Clare Perkins (pictured left) has been cast as Ava, who arrives on screen on November 20.
Mitchell added: "It's going to be mayhem. The implications will be huge. The family will want answers, and I'm sure we won't be seeing the back of Ava."