Rod Stewart has spoken with painful honesty about his marriage to model Rachel Hunter confessing that their eight-year relationship was the first in which he had been faithful to one woman.

In his sensational new book Rod: The Autobiography being serialised exclusively by The Mail on Sunday and Daily Mail the star describes how Rachel's decision to leave him threw him into four months of crippling despair, during which he had to seek therapy and lost 5.44kg.

Stewart was a 45-year-old millionaire when they met in 1990, famous for a string of glamorous girlfriends and Seventies pop anthems Maggie May and Do Ya Think I'm Sexy?

But marriage to blonde 21-year-old Hunter transformed the hard-living rocker into a devoted family man.

“In the eight years we were together, I was entirely faithful,” he writes. “This was unprecedented for me and, given my form, I don't think you'd have found too many . . . prepared to put money on that outcome. My desire to wander had simply evaporated. Rachel was everything I wanted; I became a devoted husband overnight.”

But Rachel, he now admits, was too young and felt trapped in the kind of wealthy lifestyle that involved dressing for dinner at home.

“It wasn't that she was too young for me. She was quite simply too young: too young to get married, too young to become caught up in another person's life.” He says she would rather have been ‘in her jeans, eating poached eggs on toast', like other girls in their 20s.

“It all spilled out: that she was unhappy for a while maybe for as much as a year; that she'd been trying to conceal her unhappiness, but she couldn't any more and she was going to leave me. It was like getting cracked across the back of the head with a cricket bat. I had no inkling this was coming. Not a solitary clue.

“She felt she'd entered my world as an unformed 21-year-old and been consumed by it . . . She needed to go. I was in a state of disbelief and I alternated between retreating into myself and pleading with her to change her mind. Back in LA, on my own in the house we'd shared, misery really settled in. For four months it was like some kind of 19th Century romantic fever.

“I lost 5.44kg. I felt cold all the time. I took to lying on the sofa in the day, with a blanket over me and holding a hot-water bottle. I knew then why they call it heartbroken: you can feel it in your heart. I was distracted, almost to the point of madness.” One of his attempts to seek therapy ended badly. “[She] was a middle-aged woman and what can I say? She came on to me. I'm sure there are stern paragraphs advising against that sort of thing in the professional statutes for therapists. Anyway, no, I didn't respond to her interest. Instead I got out pretty sharpish.”