LONDON — Gina Martin was at a concert in London last year, waiting excitedly for her favorite band to come on stage, when a man standing nearby with whom she had chatted briefly suddenly rubbed himself against her and placed his phone between her legs. She quickly realized that he had taken a picture of her crotch.

As disturbing as that was, the subsequent series of events left her even more shaken and incensed: the police let the perpetrator go, she said, because the photo was “not graphic” enough, showing only her underwear.

On Friday, Ms. Martin, 26, got her revenge. After months of campaigning by her, the British government said it would formally back a draft bill making “upskirting” — the practice of taking unauthorized photos under a woman’s skirt — a criminal offense. Violators would get up to two years in prison and be placed on a sex offender register.

But women’s rights advocates are already saying that, while welcome, the proposed legislation does not go far enough, punishing perpetrators only if prosecutors can prove that the main purpose of upskirting was sexual gratification or causing distress to the victim.