Sir Chris Woodhead, England's former chief inspector of schools, has died.
Sir Chris, who was aged 68, was a high-profile head of the Ofsted education watchdog between 1994 and 2000. He had been diagnosed with motor neurone disease in 2006.
His criticisms of classroom standards and "incompetent teachers" had made him a controversial figure.
But the Ofsted chief had argued: "I am paid to challenge mediocrity, failure and complacency."
Prime Minister David Cameron has tweeted: "Chris Woodhead started a crucial debate on school standards and reform. Meetings with him were never dull. My thoughts are with his family."
Education Secretary Nicky Morgan described him as an "immense figure in the world of education".
"His determination to ensure that every child had the best education possible raised aspirations and changed lives. He was someone unafraid to speak his mind or challenge established orthodoxies and our education system is the better for it," said Mrs Morgan.
The current Ofsted chief, Sir Michael Wilshaw said that when he was working as a head teacher, during Sir Chris's time running Ofsted, he had "greatly appreciated the courage and bravery he showed in confronting a complacent education establishment. He said the uncomfortable things that needed to be said".
Sir Chris was one of the most high-profile figures in education in the 1990s, identified with opposing progressive teaching methods.
He had influential roles in shaping the national curriculum and England's exam system.
He came to public prominence as head of Ofsted, where he warned about the standard of teaching. Sir Chris was Ofsted's second chief inspector, but he came to define the role of the challenging watchdog of education.
This included the controversial claim that there were 15,000 "incompetent teachers" in England's classrooms.
But when accused in 1999 by MPs of being too confrontational, Sir Chris said: "There's no point at all in having a school inspection system if it gives praise where it isn't due."
In 1999 he was also caught up in a controversy about when he had begun a relationship with a student he had taught while he had been a teacher in the 1970s. He had insisted that it had developed only after they had both left the school.
David Blunkett, education secretary during Sir Chris's last years as head of Ofsted, commended his "bravery".
"He wasn't just brave in these latter years with motor neurone disease which is a horrendous illness, but also he was brave in taking on vested interests."
Mr Blunkett said: "Occasionally we clashed, actually more often behind the scenes we agreed."
After resigning from Ofsted in 2000, Sir Chris became a professor of education at the University of Buckingham. He was awarded a knighthood in 2011.
He had spoken of the frustrations that led to his stepping down as chief inspector.
Sir Chris was diagnosed with motor neurone disease and later became patron of the campaign group Dignity in Dying.
Sarah Wootton, the group's chief executive, said Sir Chris had been "a powerful voice for an assisted dying law in his last years. Sir Chris was diagnosed with motor neurone disease, and later liver cancer and he faced both illnesses with his trademark no-nonsense courage".
Brian Lightman, general Secretary of the Association of School and College Leaders, described Sir Chris as a "controversial figure, with forthright views, and his time as chief inspector of schools frequently brought him into conflict with the teaching profession".
The leader of the National Association of Head Teachers, Russell Hobby, said: "Without doubt, Sir Chris was a significant force in education for many years."