The cause of Madonna's marriage to Guy Ritchie has been identified.

According to News of the World, her obsession with sticking strictly to her four-hour daily exercise regime took away much of the couple's time together. And despite Guy's plea that they should spend more time together, Madonna was dogged about keeping to her exercise schedule.

The last straw that broke the camel's was that the film director went for 18 months straight without sex, and they were reportedly living like a pair of siblings towards the end of their marriage.

A close friend of the couple was quoted as saying: "He got more and more frustrated as she spent nearly half the day working out. Afterwards she'd be too tired to make love.

"And towards the end Guy wouldn't be there. He'd get fed up waiting for her and go down the pub with his mates.

"He is the one behind the split."

On the rare occasions when they did get intimate, he has told pals it was like 'cuddling up to a piece of gristle'.

'All the soft feminine tones have been replaced by the build of an athlete,' he added.

The friend said that Ritchie, 40, had 'poured his heart out to him after a few drinks one evening'.

He also said that Ritchie was 'sick of taking a back seat and letting her make the decisions - so he took the bull by the horns'.

'When the cracks began to show in their marriage, Madonna wouldn't budge. Guy was pleading with her to spend more time with him, but she just wouldn't have it.

'She'd insist she did her four-hour workout and that would be half the day gone.

'He'd ask her whether she'd still be pumping iron at 60 and she'd always reply, 'Sure I will'.'