Cathy Shipton is set to leave her role as nurse Duffy on Casualty 33 years after she first appeared on the show.
The actress, 62, featured in the show's pilot episode in 1986 and has been in over 350 episodes of the long-running medical drama.
She will appear on the BBC One soap for the last time in 2020.
Her storylines throughout the years have explored important issues including sexual assault, single parenthood, mental health and HIV.
More recent episodes have charted her character's struggle with dementia.
"Walking in Duffy's shoes through the years has taught me so much for which I'll be forever grateful," she said.
"I'm honoured and humbled to have played this inspiring woman. With sadness, but a sense of completion, I say goodbye."
Simon Harper, executive producer of Casualty and Holby City, said: "Cathy's enormous contribution has not just been to Casualty but also to the national consciousness and the British television drama landscape.
"She's given us some of our most unforgettable stories and moment, not least of late in her heart-rending dementia storyline, and I simply cannot thank her enough."
Shipton will appear on screen as Duffy until early 2020.
With additional reporting by Press Association.
