For decades following the first world war, the only person arguing for the unqualified legalisation of abortion was the "aggressively unfashionable" Stella Browne. It was typical of Browne to make no pretence at objectivity; she advanced the notion that an individual woman had the right to bear or not to bear a child. This was quite above any argument about whether a woman could afford to have a baby or whether it was eugenically desirable that she should. Browne rejected half-hearted compromises such as abortion only for unmarried mothers or raped women, because "the majority of enforced and unwilling conceptions take place within the marriage tie."

Browne was loud and demonstrative and not universally loved. Colleagues such as Dora Russell were ambivalent about abortion because "we were trying to get birth control on the way and we didn't want a disturbance to our work", but she did honour Browne's intransigence  "no chairman on earth could get her to sit down." Lesley Hall, in a work of exemplary archive research, recounts the underhand means by which Browne was relegated by her enemies within the movement, so her radical ideas did not embarrass the progressives.

Her ability to put herself in the picture comes across most clearly in her statement to a parliamentary committee on abortion in 1937: "I have  I say this as a matter of public duty  the knowledge in my own person that, if abortion were necessarily fatal or injurious, I should not now be here before you." She thus publicly confessed to a crime many considered to be murder.

Before the first world war Browne joined the group around the subversive journal the Freewoman, set up by Dora Marsden and other suffragette activists to address issues of sexuality. Hall shows how Browne advanced the new forms of sexual knowledge being explored by her friend Havelock Ellis, among others, by marrying them to a feminist tradition. She wanted women to enjoy the same sexual freedom as men, and argued that there was "probably a far greater variation sexually among women than among men". To make such comments in 1912 was an assault on every established notion of gender difference in sexual behaviour.

Soon she was a speaker for the pro-contraception Malthusian League, and was later active in the British Society for the Study of Sex Psychology. Her landmark paper "The Sexual Variety and Variability of Women" of 1915 was supported by examples from the emotional and sexual life she led as a bisexual "sexual epicure". She wrote of non-vaginal sex including "the most varied, and even perverted, sex experience", and she was not above a nod to S&M, referring to the "pleasure in either inflicting a certain degree of pain on the beloved one, or suffering a certain degree of pain from them". Instead of considering it a dangerous or shameful practice, she saw masturbation as just another component in a healthy sex life.

She experienced disappointments which show that "gay boyfriend syndrome" is not new. She loved the sandal-wearing, feminist-supporting, sometimes bisexual men in her circle, but lamented that they could not give "the aggressive warm life-quickening ardour and dash and aggressiveness that are so intoxicating in a lover".

Despite her lively personal life, Browne lived in rooms with her mother and sister, doing low-grade civil service jobs, reviewing and writing for small magazines and translating. Like many activists, she was no intellectual and no analyst: she was not interested in seeing all sides of a situation, but in putting her own views across clearly and forcefully. She was "untidy, careless about her looks and appearance".

Browne joined, then left, the Communist party but never overlooked the fact that social progress requires economic progress, arguing that every household should have its own sanitary arrangements and supply of hot water. "It is not surprising that a woman with no bathroom and, of course, no separate bedroom . . . should fail to apply the pessary."

As she aged she became, if anything, more daring, denying a universal maternal instinct: "There are women to whom a man, or more than one man, mean more than any child." She argued that the need for sex is quite independent of reproductive function; and approved a suggestion that in the future technology would allow lesbians to have babies by "artificial fertilisation".

In 1935 the Sex Reform Group with which she was strongly associated published its manifesto, which included legal abortion, birth control, the simplification of divorce laws, better education and treatment for VD; the legalisation of homosexuality; and the destigmatisation of the unmarried mother and the illegitimate child. All these attitudes are now mainstream.

Browne died in 1955, 12 years before David Steel's act which legalised abortion in some circumstances, though it did not give women an absolute right to choose.

As Hall admiringly notes, she may have been "the highly neurotic Miss Browne" to other activists, but unlike many of her detractors she neither became a Tory nor converted to Roman Catholicism nor got married to a man who would be able to support her, "or in any way reneged from her existing principles". She was a wonderful woman who well deserves this excellent biography.

Jad Adams's Gandhi: Naked Ambition is out in paperback next month.