Had Dickie Mountbatten lived long enough to read Alex von Tunzelmann’s Indian Summer — a lively history of India’s partition in 1947, in which, as last viceroy, he plays a central role — a few details might have irked him. Her portrayal of his early naval career as a catalogue of catastrophic blunders, for instance, might have grated. The open discussion of Nehru’s affair with Mountbatten’s wife, Edwina, would likewise have offended his sense of decorum. One thing about Indian Summer, however, would have thrilled him: the book is to become a film. Even better, Hugh Grant is rumoured to be playing Dickie and Cate Blanchett his wife.

Mountbatten loved the movies. He and Edwina, always a handsome couple, even appeared in one during their honeymoon of 1922: Nice and Easy, starring Charlie Chaplin. Mountbatten’s life, moreover, was a constant performance. He loved the pomp and ceremony of officialdom, and in particular the brilliant white uniforms it allowed him to strut about it in. Indeed, so inherently cinematic were the lives and loves of the Mountbattens that their friend Noël Coward based the leads in In Which We Serve and Brief Encounter on them — the latter picture prompting the viceroy to exclaim “how deeply it moved me”.

So, when Working Title bought the film rights to von Tunzelmann’s book before it was published last year, it knew it was onto a winner. Not only were the central protagonists stars in their own lifetime, their story is ripe for adaptation. Indian Summer charts the rancorous partition process, in which modern India and Pakistan were created, through the intertwined lives of the five most significant characters of the time: Dickie, Edwina, Nehru, Gandhi and Jinnah. During a political moment in which squabbles over power and territory would lead to the deaths of millions, she explains how the intimate relationships between these five people — and in particular the intense love affair between Edwina and Nehru — shaped the historical moment.

“I thought, when I first came across this story, how filmic it was,” says the author, who, at 30, is now working on her second book, about the cold war in the Caribbean. “It’s a real-life Casablanca — it has huge potential.” When pressed on details about the film, though, the garrulous von Tunzelmann becomes uncharacteristically coy. She insists that, like many authors who sell the film rights to their books, she is not involved in the screenplay and “doesn’t know anything” about casting. What she means is, she’s not allowed to tell me. Not even dangling the “Well, what if you were casting it?” bait can induce her to bite.

The author’s ability to deflect questioning is, perhaps, unsurprising: her last and only boss was Jeremy Paxman. Having answered an anonymous advertisement for a “journalist’s researcher” in Cherwell, the Oxford University newspaper of which she had been editor, von Tunzelmann was interviewed by Paxman for a job. His only free moment happened to be the day of her last final, so she turned up to the interview having been attacked with the traditional glitter bomb outside the examination hall. Paxman looked at this speckled mess and bought her champagne. Von Tunzelmann was hired. She went on to research his books The Political Animal and On Royalty.

It was during her library work for On Royalty that von Tunzelmann began to become interested in Mountbatten, Edwina and Indian partition. “Whenever I read about Mountbatten, this story about Edwina’s affair with Nehru would come up, but all the historians kept brushing it away,” she says. “It was as if they were saying, ‘No serious historian would be interested in this affair.’ I kept thinking, ‘Well, actually, if two people in their situations had an affair, that was incredibly significant.’ ”

When she told Paxman that she intended to write about Mountbatten, Nehru and the rest, he told her the subject had been done to death. “No, it hasn’t,” was her reply, and, says Paxman now, “She was right.” Indeed, von Tunzelmann’s portrayal of the relationship between Edwina, a fiery, progressive woman, and Nehru, India’s great politician, is astonishing any way one looks at it.

The affair was, for obvious reasons, politically explosive. Had the world known, in 1948, of the love between the former vicereine of India (husband of a viceroy who was perceived as being markedly pro-India) and India’s first prime minister, the effect could have been to spark an all-out war between India and Pakistan. “I honestly believe the scandal would have dwarfed the abdication,” von Tunzelmann says. “It would have decided the fate of nations.”

On a personal level, the affair was also remarkable for the simple fact that Mountbatten sanctioned, even encouraged, his wife’s relationship with Nehru. In fact, he encouraged all his wife’s affairs and, when they concluded, was generous enough to offer her emotional support. In one remarkable letter, reprinted in Indian Summer, Mountbatten tries to cheer Edwina when her long-standing affair with a man named Bunny Phillips is terminated because of his engagement. “I must tell you again,” Dickie writes, “how deeply and sincerely I feel for you at this moment.”

“I think that’s a real goose-bump moment,” von Tunzelmann says. “It was the thing that really warmed me to him, actually. He knew he had to tolerate certain things about her to be with her. And he’s so generous about it. He cared enormously about Edwina, who could be quite a difficult person. He was very interested in family, but not, as far as I could make out, in sex.”

How far Edwina and Nehru’s relationship went, von Tunzelmann would not like to say. “There are only two people who know whether their relationship was sexual, and they’re both dead,” she says. “They were certainly in love. There was an extraordinary intimacy there.” A film of Indian Summer may not, she is willing to admit, follow such a nuanced approach.

If the author’s treatment of Nehru and Edwina is delicate, her portrayal of both Churchill and Gandhi is waspish. In his relationship with India, Britain’s great war hero is painted as a cantankerous bigot with little to recommend him. “I’m aware that he did some other good things in his life, but he did nothing good in terms of India,” von Tunzelmann says. “He really was a vile, vile racist. And, more important, I would hold him largely responsible for the Bengal famine, in which millions of people died unnecessarily because he would not release the grain supplies.”

The legend-bashing does not stop there. Gandhi’s saintliness starts to dissipate when one considers the strain his spartan principles placed on his family, as well as his occasionally repellent views. In May 1940, for instance, he voiced his support for Hitler; and, even when the concentration camps had been discovered in 1946, refused to resile from his opinion that, although the holocaust was “the greatest crime . . . the Jews should have offered themselves to the butcher’s knife”.

“I was surprised when I read all that stuff about Gandhi,” von Tunzelmann says. “I had started from this western position that he is basically a saint, but then you start to see it’s more complicated than that. Certainly, at the end of his life, he did some amazing things that are impossible to ignore. But still, among a lot of Muslim and Hindu groups in India, he’s a real villain figure.”

While Churchill and Gandhi receive a kicking, Mountbatten’s stock is given an unexpected boost in this account. As viceroy, von Tunzelmann argues, he was not the bumbling incompetent of his youth. Nor was he the chief author of the partition disaster, as Andrew Roberts has argued. Rather, he was an odd sort of hero.

“He’s a funny one,” von Tunzelmann says. “He’s a ridiculous figure in lots of ways — obsessed with trivial, infuriating things like protocol — but he had a progressive attitude towards postcolonial matters and he could get on with people. What you’ve got to look at is, could anyone else have done better at that particular job? I believe he was the right man and I don’t think anyone else could have done better.”

If Grant passes on the part of Mountbatten, there will be plenty of other bluff, handsome English actors waiting in the wings. The casting agents should have no problem filling the role of Edwina, either. Who wouldn’t want to play the enlightened, luscious vicereine? Nehru’s the problem. Von Tunzelmann is worried that there isn’t an English actor of Indian descent with enough “sexiness” to play him. Her solution? Look to Mumbai.

“In fact, I suggested to Working Title that they do the whole thing as a four-hour Bollywood movie, with singing and dancing,” she says. “They looked horrified. I had to explain to them that it was a joke.”

Indian Summer is published by Pocket Books at £8.99

Explore Books

• Book Extracts

• Book Reviews

• Books Group

• Audio Books

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist