Tributes have been paid to Sir Chris Woodhead, the former chief inspector of schools who had motor neurone disease.
Sir Chris, 68, led schools watchdog Ofsted from 1994 to 2000 - a tenure which included fierce clashes with teaching unions.
The former teacher resigned from his post as chief schools inspector after a series of rows with then-education secretary David Blunkett.
He was diagnosed with motor neurone disease in 2006 and, in 2009, said he would rather kill himself than die in agony from the disease.
News of his death was confirmed this morning, and was followed by tributes from friends, colleagues and politicians.
Prime minister David Cameron tweeted: "Chris Woodhead started a crucial debate on school standards and reform. Meetings with him were never dull. My thoughts are with his family."
Sir Chris once said that 4.2% of the teaching profession was not up to the job, earning him angry attacks from teaching unions.
Last November he warned that schools must become more alert to the possible sex abuse of pupils by teachers.
His comments came after Southbank International School in London he was linked to hired convicted paedophile, William Vahey, who abused dozens of boys.
Speaking in November, he said: ''There is a silver lining. It is a dreadful, appalling thing that has happened to Southbank.
"But the publicity that has been generated - if it does raise awareness in other schools, amongst other teachers, that is of course a good thing.
''Perhaps our teachers could have registered more quickly that this man was not behaving in a way that was, for want of a better word, normal.
''The signs of a potential paedophile, everyone in the teaching profession needs to know about.''
Vahey, who taught at Southbank between 2009 and 2013, was found dead in Luverne, Minnesota, in March last year. His flat in north London was raided by police the following month.
After Sir Chris's resignation in 2000, he went on to advise former Tory leader Michael Howard and worked as a columnist and author.
He was a Professor of Education at the private Buckingham University and founder of Cognita, a company running a group of private schools.
In a statement, Cognita said: "Sir Chris Woodhead was an incredibly bright, passionate and engaging man - an academic of immense stature. He was the founder of Cognita in 2004 and for 10 years guided and supported the group's development as chairman.
"Prior to Cognita, Sir Chris was instrumental in shaping education reform in the UK, leading Ofsted from 1994 to 2000 and developing a lasting inspection framework focused on high standards and school improvement.
"He was at all times determined and passionate in his work, all the more inspiring given the challenges he faced following his diagnosis with motor neurone disease.
"We are honoured to have been his colleague and our thoughts are now with his devoted wife Christine and his family."
Sir Chris, who was knighted in June 2011, did not stay out of the limelight following his resignation and often made his views on the state of UK education known.
In 2009, he denounced Ofsted as an "irrelevance" and a "waste of public money", saying the watchdog had become "part of the problem" in the education system.
And in 2013 he repeated his claim that there could be thousands of incompetent teachers at large in the UK.
Russell Hobby, general secretary of the school union NAHT said: "Without doubt, Sir Chris was a significant force in education for many years."
Education Secretary Nicky Morgan described Sir Chris as an "immense figure in the world of education".
"He was someone unafraid to speak his mind or challenge established orthodoxies and our education system is the better for it," she added.
Ofsted chief inspector, Sir Michael Wilshaw, said: "As a headteacher in the early 90s, when Sir Chris Woodhead was chief inspector, I greatly appreciated the courage and bravery he showed in confronting a complacent education establishment.
"He said the uncomfortable things that needed to be said and was tireless in his ambition for schools to be the best they could be."
David Blunkett said the contribution by Sir Chris to the debate on improvement in standards and consistency of education across the UK was "profound".
"Agree or disagree with his prescriptions for change, there is no doubt in my mind that 90% of the time he was right in believing that we should have expected much more for those youngsters who got much less from the education system than they deserved," Mr Blunkett said.
"It was always the 10% where Chris believed that he was right and much of the rest of the world were wrong that led to so many finding his style abrasive.
"His latter years had been a demonstration of bravery and tenacity in living with motor neurone disease and he should be remembered as one of the strong characters of the education world of the last 30 years."
In an interview with the Sunday Times in 2009, the former keen hiker who then had trouble dressing himself said: "The quality of one's life is more important than its quantity".
"The truth is, I would be more likely to drive myself in a wheelchair off a cliff in Cornwall than go to Dignitas and speak to a bearded social worker," he said.
"I have no immediate plans to kill myself. The progress of the disease has been mercifully gradual. I hope that I have several years of reasonable life left."
Sarah Wootton, chief executive of campaign group Dignity in Dying, of which Sir Chris was patron, said: " We are incredibly saddened to hear of the death of our Patron Sir Chris Woodhead, who became 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.
"Only last month he wrote that 'It seems to me obvious that, if the law were to be changed along the lines Lord Falconer is proposing, sufficient safeguards could be built in to protect the vulnerable. Equally, I cannot see how anybody can really believe that the state should decide the timing of my death. But we live and we die in a society... where the needs and the rights of those who could benefit from a change in the law are brushed to one side'.
"In our last conversation two weeks ago Sir Chris was looking forward to the upcoming debate in the Commons and was ready to fight for a change in the law if in his own words he 'survived' a further operation, showing his willingness to confront the stark reality of his situation.
"Unfortunately this was not to be the case and our condolences go out to his wife Christine, family and friends."
The surgery Sir Chris told friends he was due to undergo two weeks ago is believed to have been a liver operation.
In July last year, Sir Chris told Sky News he might have to starve himself to death as his health deteriorates from MND and cancer.
"I think it would be far better if I could have my death assisted in a peaceful and dignified way so that my suffering wasn't prolonged, and the suffering of those nearest and dearest to me wasn't prolonged as they had to witness the slowness of my extinction," he told Sky News.
And in February this year, Sir Chris told the Sunday Times he had found out he had colon cancer which spread to his liver last year.
He had undergone several cycles of chemotherapy and told the newspaper: "My many friends in the world of education are going to have to stick a few more pins in me."