When I asked Boris Johnson if I could write his biography, he laughed for a long time before saying: “Such is my colossal vanity that I have no intention of trying to forbid you.”

At the time, in the summer of 2004, Boris’s star shone with amazing brightness. Reputable judges predicted he would be the next Conservative prime minister.

Boris himself pointed out that the book could be “the most fantastic piss-take”. But then he asked: “What about poor Allegra?” Allegra was Boris’s first wife, although for some years he omitted her from his entry in Who’s Who.

During our third talk about the book I said I was having lunch with Allegra that very day. “Oh no!” he said. “My life is literally in your hands.”

A few days later I had lunch with a friend of his mistress, Petronella Wyatt, who told me that in 2003 she had got pregnant by Boris, and had aborted the child, but that he had told her he would look after any child she had by him.

No word of any such pregnancy had yet appeared in the newspapers, but I found that Boris was getting cold feet about the book. When I met him that evening he said in a worried tone: “If it’s a piss-take that’s okay”; but “anything that purported to tell the truth really would be intolerable”.

Some months after I started my research he offered to buy me out. As the book progressed his bids increased in size. I am inclined to think that all this must have been a protracted joke. Before the book went to press he raised the amount for a fleeting moment to £100,000.

Boris had moved from complete agreement with the book to total disagreement. He began by sounding utterly confident and ended by sounding strangely vulnerable. It was a curious transformation but also, as I was to discover, a rather characteristic one.

WHEN Boris was 14 his parents got divorced. His father Stanley broke the news to the children. Boris lined himself and his three siblings up, stepped forward and said: “Why did you have us?” Charlotte, his mother, moved into a maisonette at the top of a high, stuccoed building in Notting Hill with the four children. Her friends gathered that Stanley had been “one amazing womaniser” and she was fed up with it. According to Charlotte: “I couldn’t stay with him. He was so inaccessible, not to say completely unfaithful. I couldn’t live with him never allowing anything to be serious. That’s the essential difference between Boris and his father. I can talk to Boris about anything.”

When Boris and his siblings were with their mother, he felt he was the man in the family, who must protect her. He was tall for his age and shy, and blushed when he spoke, gazing out through his hair like Lady Diana.

This is not the Boris the public have come to know and love. A friend of the family said: “He had a dignity about him. You could see he was a very nice person. He understood what had happened to his mother, he knew it was dreadful and humiliating, and he was enduring it.”

The atrocious pain of the divorce encouraged Boris to create a mask that hid how he felt from the outside world. Many years later he confided to a woman with whom he was in love that after his parents split up he decided to make himself invulnerable.

Any sensitive adolescent might hope to do this, but few have been able to model their escape bid on quite such a father. Anyone who has met both father and son is struck by the extraordinary resemblance. They talk in an amazingly similar way, and Boris has learnt a great part of his comic art from his father. They behave like stage Englishmen, often pretending to be impossibly baffled and stupid, while behind this screen they calculate what would be to their own advantage.