Sienna Miller Reportedly Cheated On Ex-Boyfriend Rhys Ifans

Sienna Miller reportedly enjoyed several secret dates with Balthazar Getty before splitting from Rhys Ifans.

The 'Factory Girl' star was introduced to the married millionaire by Matthew Rhys - who stars with Getty in hit TV show 'Brothers and Sisters' and is Sienna's ex-boyfriend.

The pair reportedly enjoyed dinner together in Los Angeles while Sienna was filming 'GI Joe' in March, but their romance only got serious when Sienna dumped Rhys earlier this month.

A source revealed to Britain's The Sun newspaper: "Sienna has known Balthazar for a while and they've spent more time together over the last four months."

"They became closer after spending time dining with friends at the Chateau Marmont in March and April.

"When Sienna realized she had feelings for Balthazar, she ended her relationship with Rhys over the phone. She wanted to do the right thing before things really happened between her and Balthazar. Since then things have quickly got very intense between them."

A friend of Matthew - who was accused of causing Sienna and Rhys' split after he got too close to his ex during filming of 'The Edge of Love' - said: "This behavior is typical Sienna. At least Rhys Ifans can try to get over it now."

Balthazar has four children with wife Rosetta.

Rosetta is reportedly furious about the allegations he is dating Sienna and has called in divorce lawyers.

It is not known if Sienna will be named in court papers.