It was announced last month that Kim Medcalf is to return to Albert Square in the role, first played by Westbrook in 1990, but also previously played by Medcalf between 2002 and 2005.
Westbrook told The Sun: "I’ve been known as Sam Mitchell for 32 years. But nobody at EastEnders contacted me to let me know they had recast Kim in the role. There wasn’t even an email to my agent. I was shocked because they do have a responsibility to tell me and someone should have.”
The 48-year-old actor and reality TV star — who returned as Sam Mitchell in 2009 and 2016 — has publicly battled drug addiction and said that learning the part had been given to another actor could have been harmful to her mental health.
She said: “The bosses have seen me go through addiction and they have no idea where I am today.
“Thankfully I’m okay. But if I hadn’t been it could have had very serious consequences for me."
Westbrook told the Soap From The Box podcast last year how EastEnders bosses had forewarned her of their decision to recast Medcalf in the role in 2002 - and said it had been a blessing.
According to Westbrook they said: "We were dreading making the phone call. We're not offering you it, because you've been so ill the last few times you've come back.
"We do want to look after your safety first. How do you feel about it?"
She said her response was: "Do you know what? You've just taken the decision out of my hands and I feel I can put that to bed."
After Medcalf's recasting was announced Westbrook tweeted that she was considering legal action against EastEnders producers.
Westbrook made her name playing Sam Mitchell as a teenager in the BBC soap. The daughter of Peggy Mitchell and the younger sister of Phil and Grant Mitchell, she worked as a barmaid in The Queen Vic pub.
Westbrook's contract was terminated in 1996 and the star has since spoken publicly about her on-an-off battles with drug addiction.
When EastEnders wanted to bring back the character of Sam but Westbrook was unable to play her, Medcalf was cast in the role.
But in 2009 the part was written back into the BBC soap for two brief stints and Westbrook was invited to return to Albert Square.
She also appeared in 2016 for the funeral of her mother Peggy, played by the late Barbara Windsor.
In February 2019 she appeared on ITV's The Jeremy Kyle Show to discuss her drink and cocaine addiction.
She admitted: "If I had the money I would be back doing 11 bags a day."
Westbrook famously suffered damage to her nasal septum as a result of past cocaine abuse and has also previously said that a botched surgery left her with osteoporosis which is making the bones in her face weaken.
In 2018 she was diagnosed with womb cancer, but was given the all clear by doctors in April 2019.
Last year, the actor announced that she was having work done to “rebuild” her face and she has now said she is ready for the future and to get working again.
Watch: Danniella Westbrook pays tribute to her EastEnders co-star Dame Barbara Windsor