Bryan Singer -- New Sex Abuse Lawsuit ... Muscular Goon Tried Forcing Me to Screw Singer

Bryan Singer is being accused of forcing himself on a British teenager after the "Superman Returns" premiere in London.

Lawyer Jeff Herman -- who also represents Singer's other accuser Michael Egan -- is planning to file a lawsuit Monday on behalf of a John Doe ... which says Singer sexually assaulted an anonymous 17-year old boy inside a London hotel room following the "Superman" after party.

The suit says Singer and Gary Goodard -- 1 of 3 execs Egan also accused of molestation -- started to touch his client sexually ... and the teen asked them to stop.

According to the docs -- Goodard allegedly brought a "large, musclebound man" in the room to try forcing the boy into having sex by smacking him around.

The anonymous teen claims Singer molested him in a number of ways -- but they never had sex -- then the director requested the boy to sit on his chest and ejaculate on top of him. According to the docs, the teen obliged because he thought he'd be anally raped for saying no.

Goddard's lawyer says the allegations are untrue.

Bryan's lawyer Marty Singer says the same ... adding, "It is time for the media and public to focus their attention on Mr. Herman's nefarious motives and tactics which seem to be driven solely by his need to shake down an innocent man like Bryan Singer.”