•British director has two young children with model Liberty Ross
•Ross played Stewart's mother in film directed by her husband
•R-Patz recently hinted he planned to marry Kristen... and has said he 'never understood' people who cheat
By Nadia Mendoza and Sarah Bull
Kristen Stewart has cheated on Robert Pattinson with a married filmmaker twice her age, it has been claimed.
The Twilight actress, 22, is said to have enjoyed an affair with Snow White And The Huntsman director, Rupert Sanders, 41.
As Kristen prepared to spread her wings from the vampire franchise, she joined forces with the father-of-two to work on his $170m feature debut - which was released just last month.
Wife Liberty Ross, 33, played Queen Eleanor - Stewart's mother in the box office hit.
Infidelity? Us Weekly have claimed to have photographic proof that Kristen Stewart cheated on boyfriend Robert Pattinson
It has been reported that last Tuesday, Kristen was caught in the arms of Rupert - with celebrity magazine Us Weekly claiming to have photographic evidence.
Kristen can be seen wrapped in the arms of a man alleged to be Sanders on the cover of this week's issue of the American magazine.
A source told Us Weekly that Kristen and Rupert, who is married to British model and actress Liberty Ross - were caught in a romantic clinch and it seemed like 'they couldn't get enough'.
And People.com also reported the story, saying: 'Kristen is absolutely devastated. It was a mistake and a complete lapse in judgement. She wasn't having an affair with Rupert.
The not-so-happy couple? Kristen Stewart, 22, is alleged to have cheated on Robert Pattinson with married director Rupert Sanders, 41 - despite posing together on Sunday at the Teen Choice Awards
Married man: Rupert can be seen with his wedding ring at a Snow White screening in Los Angeles in May
'It was just a fleeting moment that shouldn't have happened. She never meant to hurt anyone. She's a good person who just made a bad choice.'
Insiders have made the allegations to both Us Weekly and celebrity website People.com.
Kristen's alleged infidelity came just months after Robert gave an interview to Italian Vanity Fair magazine saying that he has 'never understood' people who cheat.
He said: 'There is one thing I've never understood: Why people cheat. My parents met when my mom was 17 and my father 25, they are still together, and seem very happy.
'I grew up believing that you can be together all of your life.'
Standing by her man? Liberty, pictured here at an LA Snow White screening in May, has been tweeting rather cryptic messages
Taking direction: Kristen and Rupert seen chatting in between takes on the set of Snow White And The Huntsman
Hidden messages? Liberty retweeted a Marilyn Monroe note from Women of History, who regularly quote motivational phrases
While it is not known exactly how long the Twilight pair have been dating, due to their extreme privacy at the start their romance, R-Patz finally came clean at the BAFTAs ceremony in February 2010.
At the time, the 26-year-old told The Sun: 'It is extremely difficult but we are together, yes. We can't arrive at the same time because of the fans. It goes crazy.
'This was supposed to be a public appearance as a couple but it's impossible. We are here together and it's a public event but it's not easy. We have to do all this stuff to avoid attention.'
Life imitated art: The couple are in love onscreen too, as Edward and Bella
Happier times: The couple were photographed together during a night out in Hollywood on 19th July
Let the good times roll: The couple were seen giggling last week during a Comic-Con press conference
Just days ago, the duo looked happy at Comic-Con as they promoted the final film of their franchise, Breaking Dawn: Part 2, and also at the Teen Choice Awards on Sunday.
The news will come as a shock after British actor Pattinson hinted he wanted to marry Kristen.
Speaking to BlackBook, he explained: 'There was a magazine, with these pictures, saying I was getting married. No one ever knows what is true or what isn’t.
'Even my own mum called to ask me if it was true. It’s not. At least, not yet. But it is true that Kristen has always done something to me that others haven’t.'
And in her most recent interview, with the June issue of Elle magazine, Kristen said she was getting tired of her 'boring' life, and wanted something 'bad' to happen to her.
She said: 'You can learn so much from bad things. I feel boring. I feel like, "Why is everything so easy for me?" I can't wait for something crazy to f***ing happen to me. Just life.
'I want someone to f*** me over! Do you know what I mean?'
What will become of them? Liberty Ross and Rupert Sanders on set of Snow White And The Huntsman
Before the drama: Liberty was seen posing up with Kristen, as well as Charlize Theron and Lily Cole, at the London premiere of the movie last month
Meanwhile, Rupert's wife Liberty rather cryptically retweeted a quote from Marilyn Monroe, saying: 'Sometimes good things fall apart so better things can fall together.'
She later followed this with another retweet, this time quoting American author and poet Maya Angelou, saying: 'If you find it in your heart to care for somebody else, you will have succeeded.'
Liberty then simply wrote: 'WOW'.
Following her tweets, Liberty appeared to have deleted her Twitter account.
When Kristen signed up to play Snow White in Huntsman, many thought that it would mark a new stage in her career following her role as Bella Swan in Twilight.
And speaking about his lead actress in the role at one of the movie's premieres last month, Rupert said he was thrilled by her performance.
Dangerous liaison: Kristen kisses Ryan Reynolds in 2009's Adventureland - in which he played a married man
In action: Kristen as Snow White alongside Chris Hemsworth as the Huntsman in the movie, directed by Saunders
He told E! News: 'She's got a great spirit, she's really quite rebellious...and feisty and kind of wild. And that's really what I wanted the modern Snow White to have.
'I didn't want her to be timid and I didn't want her to be frightened and relying on men, you know.
'She's very strong and very driven, and I think that's true in her personal life...She's got a big weight of the world on her shoulders and she carries it incredibly well.'
Snow White And The Huntsman marks the first major motion picture that Rupert directed - but he has already signed on to take the helm of the sequel.
He has said: 'Our story is a 'once upon a time', but it's not a "happy ever after."'
It is believed that Huntsman was the first in a planned trilogy of movies about the modern Snow White.
Centre of attention: Kristen spoke at the Teen Choice Awards, flanked by co-stars R-Patz and Taylor Lautner
Fact or fiction? Rupert posed with Kristen and Chris Hemsworth at the Australian Snow White premiere
Prior to the film, Rupert had made a name for himself directing commercials.
Following the infidelity claims breaking, Kristen is already facing backlash from fans of the tween favorite couple.
One fan said: 'HOW COULD SHE? Rob loves her so much.'
Another told HollywoodLife.com: 'Ever since she did Snow White And The Huntsman she changed. She’s more Hollywood now. More into her looks. I mean how many magazine covers she done in the past few months? I lost count.'
Distracted: Kristen looked deep in thought as she played golf in Pacific Palisades on July 20th
Others took to Twitter to voice their disappointment in the actress, with one tweeting: ' OH HELLLLL NO. KRISTEN STEWART CHEATED ON MAH BEW WITH SOME DIRECTOR.
SHE'S SUPPOSED TO BE GOOD TO ROBERT PATTINSON, his luscious self.'
And another added: 'Kristen Stewart, how could you cheat on Robert Pattinson with a married man that has children? So disappointed in you.'
In a bid to lose her wholesome image, Kristen also played the role of Emily in Adventureland in 2009.
The cheeky actress, who has disrobed for her latest project On The Road, engaged in sexual scenes with Ryan Reynolds, who played the married park maintenance man.
MailOnline is awaiting comments from Kristen's representatives.