In the second volume of A Life of Picasso, John Richardson also argues that Picasso, born in the 19th century into a traditional Andalusian family, has been unfairly judged against the mores of the late 20th century while other chauvinists, such as Rembrandt and Matisse, have been spared.
The book will fuel controversy about the artist's life, which is currently the subject of a Merchant Ivory film, Surviving Picasso, starring Anthony Hopkins and based on Arianna Stassinopoulos's hostile biography, Picasso: Creator and Destroyer.
"It is certainly true," Richardson said yesterday, "that Picasso treated women badly, but he also showed great compassion and tenderness."
During the First World War, he said, the Spanish artist fell in love with two women who abandoned well-advanced plans for marriage with him, leaving the legendary seducer devastated. The biography devotes a chapter to each of them.
Gaby Lespinasse was the focus of his desires in 1915. "She was a very sweet girl who is thought to have danced in the Montparnasse cabaret," Richardson says.
"Her lover was Herbert Lespinasse, whom she subsequently married, and who was one of the creators of St Tropez. He had a house there, which was a centre of Bohemian life. Picasso and Gaby went there, like many others, to escape Paris during the war. They used it as a love nest.
"There are many beautiful watercolours of the rooms by the Mediterranean, that are unique in Picasso's work. On the same sheets, there are love letters and descriptions of their bedroom.
"He gave her all these, but they did not come to light until recently. They are very touching. A lot have the names of Gaby and Picasso entwined in different colours. They merge into one, their names, as it were, copulating. There is another letter, in which Picasso says 'I love you' in every different colour. Here you can see the sweet and tender side of Picasso.
"He had met Gaby while his mistress of the period, Eva, was dying of cancer. She was very young and beautiful. I don't know how he explained to her about his trips to St Tropez, because in November and December 1915 he was going almost every day to see Eva.
"When Eva died, Picasso assumed that he would marry Gaby. But she decided to marry Herbert instead. She felt she would have a better life with him than with a great painter who was known to be possessive and difficult.
"So, on the rebound, in the spring of 1916, Picasso fell madly in love with Irene Lagut. He and a friend, the poet, Guillaume Apollinaire, abducted her. They took her to a villa in the Paris suburbs. But Picasso didn't fasten the shutters well and she escaped, although she came back of her own accord a week later.
"The affair was on and off until the end of 1916, when they decided to get married. Then at the last minute, when they were going to meet family in Barcelona, she returned to her previous lover in Paris. Irene was basically a lesbian. That is why she went back and forwards between girlfriends and then boyfriends. She led a peculiar life - she had been kept by a Russian grandduke in Moscow."
She did, however, become Picasso's mistress again in 1923 and one of Picasso's most famous works, The Lovers (1923), showing a young man and a woman, is, reveals Richardson, of the couple.
"Irene recently died in an old people's home, aged 101," he said, "but a friend did meet her and like many old ladies with disreputable pasts, she denied all the stories. Fortunately, I found her letters in an archive in Florence."
The consequence of Picasso's second rejection, Richardson says, was that Picasso went in search once more of a wife. In spring 1917 he went to Rome to work with the Diaghilev Ballet, whose wartime headquarters were in Rome. There he met the Russian ballerina, Olga Kho-khlova, whom he married in 1918 when he was 37.
He eventually tired of Olga, who introduced him to a stifling bourgeois lifestyle of middle-class conformism and order. There were to be many more mistresses, including Francoise Gilot, who famously remarked that Picasso treated all women like goddesses and then as doormats. She did not mind being a goddess, but she drew the line at the doormat.
"Picasso's feelings for women were extremely intense," Richardson said. "He could not function without a woman around. Dora Maar, his mistress from 1936 to 1944, told me that when women in Picasso's life changed, everything changed: the style of painting changed, the band of friends changed, the poet - he always had a poet around - the house and the dog all changed.
"It is not strictly true, but it is quite true. And his relationship with women is reflected in his work. If he is tender with them, there is tenderness in his work. If a woman is sick, you see it, as in portraits of his second wife, Jacqueline, who was frequently ill. And when women are replaced, you might have a painting with dark hair on one side and blonde on the other, so that a woman can see for herself that she is being replaced."
Picasso's women do not always appear figuratively. "I have found that after 1910 Picasso paints his mistresses not as conventional figures, but perhaps as a guitar or an instrument that could be played," Richardson said.
In one previously unknown work found in Russia, small letters are scratched in a dark corner of the painting. They are "Eva" representing Eva Gonel, his mistress at the time.
8 A Life of Picasso, vol II, 1907-1917: The painter of modern life by John Richardson; published next month by Jonathan Cape; pounds 30.Reuse content