The manuscript, in its firm, clear hand, twists and weaves a path around a large proportion of passages that are heavily scored through and cancelled. Many other paragraphs are littered with minor excisions and interpolations, altering pronouns and changing the tenses of verbs. Preserved in the British Library, in a green leather binding, its outwardly sedate appearance seems to belie the fierce expression of its contents. For Florence Nightingale's Cassandra , written in the period immediately preceding her departure for the Crimean War, in 1854, is a ferocious attack on that most sacred of Victorian institutions: the family.

The bitterness of feeling that lies behind Cassandra derived straightforwardly enough from Nightingale's own experience as an upper middle-class daughter of extraordinary intellectual gifts, who saw her ambition to follow a vocation as a nurse thwarted for many years by the opposition of her parents and sister.

To Nightingale, frustration at her own plight was mirrored in the experience of other women of her class in mid-19th century Britain. These women, she wrote, could find no outlet in "a cold and oppressive conventional atmosphere" to satisfy their "Passion, intellect" and "moral activity". They are "never supposed to have any occupation of sufficient importance not to be interrupted", and so fritter away their days in looking at prints, doing worsted work, reading out loud, and taking drives in the carriage. At night they pay the price for their inactivity: "the accumulation of nervous energy ... makes them feel ... when they go to bed, as if they were going mad".

Virginia Woolf, reading Cassandra on first publication at the end of the 1920s, thought it more like screaming than writing. JS Mill, sent a privately printed edition of the essay nearly 70 years earlier, had been less interested in its emotional temperature than in its application to his own theories on the condition of women. He was especially drawn by its depiction of family as an instrument of tyranny. Nightingale had provided valuable first-hand testimony to back up his own indict ment of the family in the book he was then working on, The Subjection of Women .

Woolf also drew on Nightingale's work in A Room of One's Own, her seminal analysis of the historical relations between women and fiction, particularly in her discussion of the vital necessity of time and privacy for women's creative work. Woolf would have read Cassandra when it was published in 1928 as an appendix to The Cause , the history of the British women's movement by Ray Strachey, a leading suffragist (and, ironically, sister-in-law of Lytton Strachey whose Eminent Victorians , a decade earlier, had rocked the sentimental, plaster saint, image of the "Lady with the Lamp").

Over the years Cassandra has maintained its position as a focal feminist text, an important documentary link between women's earlier struggles for personal, legal, and political liberties, and the full-blown fight for emancipation that emerged in the first decade of the 20th century. Yet its complex evolution is relatively unknown. The different manuscripts in the British Library reveal how Cassandra survives in various guises: as a dialogue between two daughters and their parents, as an autobiographical novel and, finally, in the form published by Strachey, as a rather broken-backed, generalised third-person essay.

The alterations that Nightingale made to the text reflect the changes to her own life and circumstances, as well as to her personal image of herself. The mysteriously exotic setting of the novel version, with its accent on phantoms and enchantment, are a by-product of the world of dreaming - once described by Nightingale as "like gin-drinking" - which provided the younger Florence with an important escape from the humdrum concerns of family routine.

The later recension in impersonal essay form, revised in the years following her return from the Crimea in 1856, feels as if it has had the life bled out of it, and provides a striking illustration of Nightingale's transformation into a woman of power - but of a type of power which craves anonymity, for religious motives as well as reasons of gender, and which walks invisible, working behind the scenes, to achieve its ends.

We can be fairly certain therefore that Florence Nightingale would not have welcomed any publication of the different versions of Cassandra. Indeed, when Benjamin Jowett, Master of Balliol College, Oxford - and a man who may have been briefly a suitor - guessed at the close autobiographical inspiration that lay behind the piece, and suggested that she herself was Cassandra, the cursed prophetess of Greek myth, Nightingale baulked at the identification.

Despite the essay's title, there is no Cassandra in the published editions we know today. In the manuscript of the original novel, however, the tragic heroine Nofriani, who represents Nightingale's own situation as a woman suffering through a life of confinement and enforced idleness, rhetorically renames herself after the doomed prophetess: "Oh! call me no more Nofriani, call me Cassandra. For I have preached & prophesied in vain. I have gone about crying all these many years, Wo to the people! And no one has listened or believed. And now I cry, Wo to myself! For upon me the destruction has come."

Nofriani dies at 30, Nightingale's age in 1850-51 at the time of writing. Her final words welcome the freedom that only death can provide.

To read Nightingale's letters to her family from this period, full of affection and dutiful concern, and only occasionally spiked with cynicism, is to realise the extent to which she felt forced to suppress her profound, almost suicidal despair. "As long as the iron chain is drawn tight round the family, fettering those together who are not joined to one another by any sympathy or common pursuits, it must be so ..." In these circumstances, her need to channel her anger and sense of injustice into a work of fiction is easily understandable.

However, Nightingale's new status as an internationally famous real-life heroine, several years later, drastically altered the form in which Cassandra could be offered to the public. She still wanted to publish it, as part of her monumental philosophical work Suggestions for Thought , though as an anonymously narrated commentary on the lives of women everywhere, rather than as an account of the personal suffering from which she had managed to liberate herself.

To this end she eliminated the imaginative elements, all the compensatory fantasy, and refashioned Cassandra as something that might prove more palatable to a general audience, of male as well as female readers. But, unexpectedly, the removal of the fictional devices served only to emphasise the autobiographical origins of the piece. Without the distancing mechanism of fiction, the author was left exposed. Immersed in countless other projects, Nightingale put the essay to one side.

What impact would Cassandra have made had it been published at this point? By the late 1850s, debates on the Woman Question were beginning to shift ground in the area of women's physical and intellectual capacity to enjoy some sort of freedom from domestic restraints. In 1858, Emily Shirreff's Intellectual Education and its Influence on the Character and Happiness of Women unknowingly rehearsed one of Cassandra 's most arresting contentions about the aggravating effect of a retiring, sedentary life on the mental health of women, arguing forcefully that many forms of "mental suffering now frequent among them" would actually be lessened if they were exposed "to the toils and hardships of a man's life".

But aware of the growing number of women who wished to escape from their "vacant, listless" lives by following "the example of Miss Nightingale", Shirreff couldn't help but make the point that Florence Nightingale was almost too remarkable to be imitated by other members of her sex.

· Mark Bostridge is writing a biography of Florence Nightingale. The Florence Nightingale Museum is at St Thomas' Hospital, London SE1. Tel. 020 7620 0374. Web www.florence-nightingale.co.uk