George Clooney has asked Matt Damon to be his best man.

The 53-year-old star proposed to his girlfriend Amal Alamuddin last month and although rumors had suggested he was planning to ask Brad Pitt to be his best man, it's now been revealed that he's chosen his The Monuments Men co-star instead.

"A lot of people thought Brad was a certainty to be George's best man as they have been such good friends for years. But he is probably even closer to Matt. Besides many years ago, during one drunken night together, George made a promise to Matt that he would make him his best man if he ever got married - so Matt called to claim his prize," a source said.

However, as 50-year-old Brad worked with Matt, 43, and George on Ocean's Eleven and its sequels, its thought that the hunk will be there to support both of his pals. "Brad took the news with good grace - but he did say he'd heckle Matt during his speech," the source added.

Meanwhile, George is reportedly planning to throw his British lawyer fiancee the wedding of her dreams either at his home in Lake Como, Italy or Cabo San Lucas in Mexico, before relocating to London so that she can continue her work as a barrister at the Doughty Street Chambers.