Fans of Blue Lights got the shock of their life when they tuned in for season two and spotted the actor very much alive and kicking, but this time in a new role as a retired police officer.

PC Robin Graham was introduced at the very end of the episode when a trainee solicitor turned up on his doorstep investigating a decades-old case he was involved in.

Although Robin begged her to leave the past alone, the scene sets up a storyline where her digging drags him back into the limelight.

Astounded viewers flocked to social media to share their thoughts on seeing the recognisable actor in a completely different series.

One baffled person questioned: “Is that Charlie from Casualty??”

Another mused: “I have watched Derek Thompson as Charlie for so long that it will be strange seeing him in a different role.”

A third agreed: “Weird seeing Derek in something else.”

A fourth joked: “So that’s where Charlie Fairhead retired to after the NHS!”

Derek previously admitted he wasted no time in saying yes to signing up for the programme amid his Casualty exit.

He 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.”