Idris Elba -- Sends Wire Message ... Not My Dong

"Wire" star Idris Elba actually had a wire in his pants this week ... but sadly it was a real wire.

Elba got a slew of new fans who don't even watch TV but saw the bulge that extended to his knee ... while filming a movie in London.

But the 41-year old fessed up Saturday and made it clear ... he was flattered: "The good news is i got a s**t load of followers. The bad news is, that is a mic wire."

He adds, "#egowentintospaceshipmodethough"