Singer Madonna and her director husband Guy Ritchie arrive on the red carpet at the 61st Cannes Film Festival May 21, 2008. [Agencies]

Madonna and Guy Ritchie are having marriage problems because they are so competitive, claims the singer's brother.

Christopher Ciccone - who has released a book about the pop superstar, 'Life With My Sister Madonna' - believes director Guy's lack of success in recent years has caused tensions within their relationship.

He told UK TV show 'GMTV': "Being married to her can't possibly be easy. I know what it's like living with her and growing up with her. As far as they're concerned - I don't know anything one way or the other, if they're doing well or not but I know that things are not as good as they could be. "I think it is because her career sort of continues its gentle glide up, where Guy's has sort of gotten a little bit stale.

And they're both very ambitious people and competitive, so it has to be little difficult." Christopher insists that anyone who marries his sister is in for a tough time because she is so ambitious.

When asked why being married to Madonna would be difficult, he said: "First of all you would have the difficulties that a normal marriage would have. Then it's all public. And then it's two very ambitious and creative people competing.

On some level they're going to be competing with each other and if one is doing better - I mean there's a major machine behind it before they got married."