EastEnders executive producer Clenshaw said of Cassidy’s departure: “Over the last 31 years in EastEnders, Natalie has been involved in some of the show’s most iconic and explosive storylines.
“Her portrayal of Sonia has made her a firm fan favourite amongst the viewers.
“I won’t be revealing the [minute] details relating to her exit, but I can say that she will take a central role in the 40th anniversary storylines in a week of truly unmissable drama that will change the lives of those in Walford forever.
“I’d like to thank Natalie for her time at EastEnders, and I wish her all the best for the future.”
Sonia is well known for her poor trumpet playing, which angered her family and neighbours, during her early days on the soap.
Some of her biggest storylines include her being bullied by her friend Clare Bates (Gemma Bissix) after she fell in with the wrong crowd at school, and her teenage pregnancy.
The soap’s 40th anniversary will also see the return of Grant Mitchell (Ross Kemp) after a near 10-year absence.
On Friday, the BBC also confirmed it would air a live episode as part of the soap’s 40th anniversary celebrations.
The live episode will feature the result of an interactive twist, which will see viewers have power to decide the outcome of Denise Fox’s love story, and whether her future should be with Jack or Ravi.
The events of the anniversary week episodes will take place over one day in Albert Square, with an hour-long episode airing on 19 February, followed by the live episode the following night.
