The starlet made a public statement admitting that her "momentary indiscretion" had "jeopardised" their relationship.
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Her statement read: "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."
Since meeting on the Twilight series nearly four years ago, the 22-year-old actress and her gorgeous Englishman have been dating discreetly and were revered as a couple by the so-called 'Twihards'.
Their relationship was rocked earlier this week by pictures of the fantasy heroine getting close to film maker Rupert Sanders who is married with two children.
Her statement was quickly followed by a similar apology from 41-year-old Rupert to his wife, Vogue model Liberty Ross (above right), saying he was "utterly distraught about the pain" he had caused his family.
The director added: "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.'
In the adventure Snow White And The Huntsman, Kristen plays the eponymous female character while Liberty is cast as her mother.
Rupert and Liberty's children, Skyla Lily Lake, seven, and Tennyson, five, also appeared in the movie.
Before the relevations, the 34-year-old covergirl was singing the praises of the film's leading lady.
"[Kristen] was my number one choice," she told Celebuzz at the movie's London premiere in May. "I think there is no better Snow White in my opinion. She is great. I mean, I've never watched a Twilight movie, but I loved her in Sean Penn's film just for that moment that she's in it.
"Ever since then — she's got something very transfixing about her. We needed a modern, edgy, cool girl."