Ruth Perry died while waiting for a report that downgraded her school.

Prof Julia Waters was speaking to members of a head teachers' union which is also asking members who work as Ofsted inspectors to consider quitting.

Ofsted said most school leaders thought inspections were "constructive and collaborative".

It comes as a school in Cambridge is set to challenge Ofsted in court for not following correct procedures, after an inspection left its head teacher feeling suicidal.

In an emotional address at the National Association of Head Teachers (NAHT) conference, Prof Waters said a "terrible injustice" had been done to her sister, and she would not "give up until Ofsted is radically reformed".

Ms Perry took her own life in January, weeks after an Ofsted inspection. The Ofsted report, published after her death, downgraded her school's rating from Outstanding to Inadequate - going from the top to the bottom of the grading scale.

"The injustice of that one-word judgement destroyed Ruth's career, her world and her sense of self," Prof Waters said.

She said head teachers who served as inspectors were "no doubt" doing their "best", but were working with "a flawed, inhumane system".

She called on them to remove references to Ofsted grades from their websites and schools.

"Stand up to Ofsted, speak out, take down your banners, hand in your badges," she said. "Ofsted must be reformed."

The NAHT also passed an emergency motion asking members to "consider refraining from carrying out inspections".

Ruth Perry, who took her own life in January, had been waiting for an Ofsted report rating her school as "inadequate"

Prof Waters previously told the BBC that Ofsted inspections should be paused, so that an independent inquiry into what happened at Ms Perry's school, in Reading, could take place - as well as a review of the culture of inspections at Ofsted.

Last week, Amanda Spielman, Ofsted's chief inspector, told the BBC's Sunday with Laura Kuenssberg programme that she believed the findings of last year's inspection were "secure".

She said the inspection team had "worked with the professionalism and sensitivity that I would expect".

Asked if she had concerns about what happened, she replied: "From what I've seen, I don't have any reason to doubt the inspection."

Prof Waters said Ms Spielman's response was "totally inadequate".

Ofsted has described Ms Perry's death as a tragedy.

A spokeswoman for Ofsted said their inspections were "first and foremost for children and their parents - looking in depth at the quality of education, behaviour, and how well and safely schools are run".

"We always want inspections to be constructive and collaborative and in the vast majority of cases school leaders agree that they are," she added.