Given that they have been dating for less than a year, talk of a summer wedding for Sienna Miller and her boyfriend Rhys Ifans always seemed a little premature.

Now it seems that marriage could be off the cards altogether.

Friends say the couple are going through "their first serious rough patch" and are desperately trying to repair the fault lines that have opened between them.

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Rhys Ifans and Sienna Miller enjoy the sun in West London yesterday

Much of the strain has come from trying to maintain a long-distance relationship: Sienna, 26, has been filming GI Joe in Europe while Rhys, 39, has been working on the Richard Curtis movie Radio Caroline in Southampton.

The couple were reunited last week in Los Angeles, having not seen each other for several weeks, and I'm told they spent much of their time discussing their future.

"It has been very strained and things are very sensitive at the moment," discloses a friend. "They are not over, but Sienna has made some tearful long-distance calls to Rhys and has confided to some of the cast on her movie that things aren't great."

I understand Sienna's frequent jetsetting trips to LA have triggered some of the rows, as has her close friendship with actor Sean Penn, who was photographed with her at the Coachella music festival last month.

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Sienna Miller's friendship with Sean Penn, with whom she is pictured at the Coachella Festival in California, has caused friction

"Rhys can be quite jealous and he doesn't like Sienna hanging out with other guys, especially Sean who she seems to have a serious crush on," my source adds.

"She also had lunch with Matthew Rhys in LA in March. They used to date and Rhys is very insecure about him. He wasn't happy they'd met up."

When Sienna returned to the UK several weeks ago, she upset Rhys by taking a few days holiday with her girlfriends instead of spending time with him.

"It was a bit cruel," says my source. "It seems Sienna is the one wanting to cool things a bit."

A spokesman for Sienna refused to comment.