Derek Thompson will be appearing in the second series of Blue Lights.
The Belfast-born actor, 75, has been the longest-serving cast member on Casualty.
He played nurse Charlie Fairhead for almost four decades and around 900 episodes of the soap before the BBC announced last year that he would leave in 2024.
Thompson will guest star as retired police officer Robin Graham in the second series of Blue Lights.
Graham will be introduced as someone whose “past and conscience catch up with him” when trainee solicitor Jen Robinson (Hannah McClean) investigates an old case.
Thompson said: “I loved the first series of Blue Lights and thought, as did the majority of my friends and colleagues, that it was the best new TV series of last year. I was thrilled and excited (star jumps in the living room) when I got the invitation to take part in series two.
“I was born in Belfast and was equally excited (more star jumps) to travel over to work on the show and pleased to report, having read the scripts, that I’m sure the fans will love the second series every bit as much as the first.
“Greatly looking forward to it myself as I’m one of them.”
Blue Lights, written by Declan Lawn and Adam Patterson and launched on BBC One and iPlayer in March 2023, has been a critical success.
The corporation has commissioned a further two series of the Belfast-based police drama from Two Cities Television, before the second series airs this spring.
The Two Cities team said: “We are delighted to have Derek Thompson back working with us in his native Belfast; it is an honour to have him as part of the Blue Lights family.”