Ugly Betty star America Ferrera has laughed off reports she is engaged to longtime beau Ryan Piers Williams.

The star - who plays the hit show's lead character Betty - was at the centre of online speculation suggesting she was set to wed her filmmaker boyfriend after she was spotted sporting a shining diamond ring on her engagement finger on the set of the ABC show earlier this week.

But the actress' representative has denied the reports, according to American website Usmagazine.com, apparently suggesting the ring could be part of a plot twist on the hit show.

They couple met as students when they worked on a film while studying media at the University of Southern California.