Ex-Voice of Elmo Kevin Clash -- Third Underage Sex Accuser Threatens Lawsuit

Kevin Clash is being accused of a third sexual relationship with an underage boy -- and the former voice of Elmo could be sued as early as Tuesday, according to a report.

The latest accuser claims he was 16 when he engaged in "sexual activity" with Clash back in 2000 -- and alleges they met on a gay chat line.

It's a very similar claim to the one made by accuser #2, Cecil Singleton. TMZ broke the story when Singleton filed suit last week -- and a few hours later Clash resigned from "Sesame Street."

The third accuser's attorney, Jeff Herman, told the NY Daily News his client plans to file a lawsuit in Manhattan Federal Court as "John Doe" ... because he doesn't want to go public.