The actor, who played Barry Evans in EastEnders, has started filming two episodes to air in August.
He will play gang leader Dave Monks, who comes under scrutiny during an international drugs smuggling investigation.
DS Max Carter (Christopher Fox) works under cover and infiltrates Monks's family.
Speaking about his role, Williamson said: "I had forgotten about the long days and how hard the actors and crew work on a continuing drama, but everyone has been fantastic to work with.
"I am really chuffed with the character and storyline I have been given and playing a villain is something I want to do more of!"