Photos of Kristen and the 41-year-old married Snow White And The Huntsman director published in US Weekly magazine have shocked fans of the couple worldwide.
'I'm deeply sorry for the hurt and embarrassment I've caused to those close to me and everyone this has affected,' says Kristen, 22.
'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.'
R-Pattz and Kristen, who met while playing the roles of Bella Swan and Edward Cullen in the Twilight films, sparked rumours of an engagement earlier this month when they were spotted shopping for rings at West Hollywood jewellery store Neil Lane.
Though Rob, 26, has denied he's proposed, it's clear his feelings for Kristen run deep.
'It is true that Kristen has always done something to me,' he told BlackBook magazine recently, 'that others haven't.'
Where to next?
- Rob and Kristen: Best red carpet moments