Viewers watching the BBC's News at Ten on Monday evening (local time) were exposed to a woman's breasts on a computer screen during a live report by presenter Sophie Raworth.
A BBC employee wearing headphones and slumped in his chair was spotted watching the woman undress in the raunchy scene.
Ms Raworth was presenting a live piece on the England cricket team's victory over South Africa when viewers noticed the screen behind her. Approximately 3.8 million people were watching the show.
One person tweeted: "Why is there a porn video playing behind one of your news anchors during a report?"
The scene is believed to be from an action movie, not porn.
A television source told The Sun: "This has gone down like a lead balloon at the BBC. It is totally unprofessional.
"It beggars belief a sex scene should be played live on air. The employee is on borrowed time when bosses catch up with him."
Viewer Lyndsey Robinson noticed the man watching the clip and posted a video of it on her Facebook page and wrote: "Did anyone else notice news last night on BBC??? Thought I was seeing things...but just re-wached On iplayer...nope I definitly saw what I thought I had [sic]."
The BBC is investigating the incident.