Former chief inspector of schools Sir Chris Woodhead, who had motor neurone disease, has died, friends said today.

The 68-year-old, who was the top schools watchdog for six years until 2000 and had some fierce clashes with teaching unions, was diagnosed with the disease in 2006.

He resigned as chief schools inspector after a series of rows with then-education secretary David Blunkett.

Sir Chris said in 2009 that he would rather kill himself than die in agony from the disease.

He told the Sunday Times then that, as a former keen hiker who now walks with a stick and has trouble dressing himself, 'the quality of one's life is more important than its quantity'.

He said he had ruled out travelling to an assisted suicide clinic like Dignitas in Switzerland.

'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.'

Sir Chris once said 4.2% of the teaching profession was not up to the job, earning him angry attacks from teaching unions.

Prime Minister David Cameron paid tribute, tweeting: '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 was in the news last November when he warned that schools must become more alert to possible sex abuse of pupils by teachers, after an elite independent school he was linked to hired a convicted paedophile who abused dozens of boys.

An independent report criticised a blase attitude by staff at the Southbank International School in London which allowed convicted American child molester William Vahey to commit sickening crimes while teaching there for four years.

Sir Chris, chairman of the school board, urged other schools to learn from its mistakes and follow its lead in toughening child safety procedures, saying staff have to be 'constantly vigilant'.

The report by Hugh Davies QC also suggested that Vahey, who was convicted of child molestation in California in 1969, subsequently enjoyed a career spanning 40 years and schools on several continents because no-one checked his US criminal record.

The pervert committed suicide in March last year aged 64, shortly after the FBI started investigating pornographic images on a thumbnail drive of at least 90 boys aged from 12 to 14, who appeared to be drugged and unconscious.

Sir Chris said in November: 'You just cannot trust the initial (background) checks. You have got, as a school, to be eternally vigilant without being paranoid and destroying the culture of trust in the school.

'It is a fine line but any school, particularly an international school, that thinks that it can rely on the initial checks, is kidding itself.'

He added: '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, on March 21 last year. The following month his flat in north London was raided by police.

In May last year the FBI revealed that it had been contacted by 'several hundred' people either alleging they were abused by Vahey or providing information for the police investigation.

The teacher, who was hired by former principal Terry Hedger, ran Southbank's Travel Club and images of between 50 and 60 students at the school, apparently taken on school trips, were found by police on his computer drive.

Mr Davies' report found 'serious failures in individual decision-making', with inappropriate conduct either under-reported or inadequately recorded and managed.

He described a 'false but dominant assumption by senior decision-makers that teachers, once vetted, could not represent a risk to students ... and that such criminality would not occur at Southbank'.

He also said senior staff did not understand the patterns of institutional sexual offending against children, with an 'incoherent' system for reporting abuse.

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 chairman of Cognita - a company running a group of private schools.

The former teacher, who was knighted in June 2011, did not stay out of the limelight, often making his views on the current state of UK education known.

In 2009, he denounced Ofsted as an 'irrelevance' and 'a waste of public money', saying the watchdog had contributed very little to schools' understanding of their task and 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.