Sienna Miller and Balthazar Getty have sparked rumours that they are having an affair.

Allegations have surfaced that the English actress has been spending an inappropriate amount of time with the 'Brothers and Sisters' star, who is married to fashion designer Rosetta Millington.

A source said: "Sienna's known Balthazar a while and they've spent more time together over the past four months.

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

"But when Sienna realised she had feelings for Balthazar she ended her relationship with Rhys [Ifans] 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."

Sienna and the great-grandson of Jean Paul Getty, founder of the Getty Oil Company, were introduced through her ex-boyfriend Matthew Rhys, who plays Kevin alongside Getty's character Tommy, in the hit US TV series.

The controversial twosome have been seen together at a petrol station, a hotel and a restaurant, and the news has allegedly led Rosetta to flee to Italy with their four children.