The 22-year-old star was photographed in a passionate embrace with her 41-year-old married Snow White And The Huntsman director Rupert Sanders despite her relationship with Twilight Saga co-star Pattinson. She has released a statement admitting her guilt.

Stewart said: "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 jeopardised 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."

Sources close to the actress insist she wasn't having an affair with Sanders and it was a fleeting mistake.

An insider said: "Kristen is absolutely devastated. It was a mistake and a complete lapse in judgment."

Stewart and the 26-year-old Pattinson have been dating since meeting on the set of Twilight four years ago, in which they play vampire Edward Cullen and his love interest Bella Swan.

Another report says Pattinson has been left "heartbroken and humiliated".

The report says the actor was stunned when he discovered the news and sources say he is struggling to cope.

A source who saw the pair fighting backstage at the Teen Choice Awards in Los Angeles on Sunday said: "Rob and Kristen were having a long and intense conversation backstage, and he looked miserable, heartbroken and humiliated. She was pleading."

Another source added to the New York Post newspaper: "Rob is deeply in love with Kristen and is very jealous when other men hit on her. This will crush him."

Meanwhile, another report says Sanders is "utterly distraught" about cheating on his wife with Stewart.

The director and his leading lady were photographed in a series of intimate embraces in Los Angeles last week and Rupert, who has two children with wife Liberty Ross, has released a statement admitting his guilt.

He said: "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."