Stewart met Pattinson on the set of the vampire film franchise in 2008 and their onscreen romance quickly spilled over into real life.
But on Wednesday, just days after making a joint appearance at the Teen Choice Awards on Sunday (22Jul12), new images of the actress cuddling up to Sanders, who has two children with his model wife Liberty Ross, were published in Us Weekly magazine, causing widespread speculation about a potential affair.
And now the 22 year old has confessed to cheating on Pattinson - her boyfriend of more than two years.
A statement released to People.com reads, ""I'm deeply sorry for the hurt and embarrassment I've caused to those close to me and everyone this has affected. This momentary indiscretion has jeopardized the most important thing in my life, the person I love and respect the most, Rob. I love him, I love him, I'm so sorry.""
Sanders has also issued a public apology, stating, ""I am utterly distraught about the pain I have caused my family. My beautiful wife and heavenly children are all I have in this world. I love them with all my heart. I am praying that we can get through this together.""
Ross, who ironically played Stewart's mother in the movie, has since alluded to her heartbreak in a series of posts on Twitter.com, writing, ""Wow...Sometimes good things fall apart so better things can fall together.""
Pattinson has yet to comment, but during a recent interview with Italy's Vanity Fair magazine, he tellingly spoke of his inability to ever cheat on a loved one: ""Why do people cheat? I can understand the impulse. But not how you can keep two relationships going at the same time for long.""
He added, ""I'm not the casual-affair kind of guy. If I choose to be with someone, it's because I really want it. When I have a relationship, I'm 100 per cent into it.""
While the future of the couple's relationship remains unclear, Stewart and Pattinson will be forced to reunite in November (12) for promotional duties surrounding the final installment of the Twilight series, The Twilight Saga: Breaking Dawn - Part 2.