Williamson, who played Barry Evans on the BBC show for ten years until 2004, declined to name the stars that he caught out, but he revealed that it was a "shocking" discovery.
During a magazine interview, he said: "When I worked on EastEnders I heard a couple having sex in the next dressing room.
"I hung around the studio to see who would come out, and let’s just say it was quite shocking. Both were on the show."
It has not been confirmed who Williamson was talking about, but during his time on the soap he worked alongside actresses Gillian Taylforth (Kathy Beale), Tamzin Outhwaite (Melanie Owen) and Charlie Brooks (Janine Butcher), who played his on-screen wife.