The day after he told the BBC that doctors often resisted change that would benefit patients, Tony Blair instructed teachers to end the "culture of excuses".

He said a mindset existed within teaching which was "one of the most powerful forces of conservatism in our society".

His attack provoked an immediate, irked reaction from union leaders.

Nigel de Gruchy of the NASUWT teachers' union said: "It's rather unedifying to see the prime minister thrashing about blaming unspecified numbers of teachers and doctors."

The prime minister addressed hundreds of newly-appointed headteachers on Thursday morning - the day after a survey showed one in five advertised vacancies for heads in England and Wales could not be filled.

The BBC's Sue Littlemore reports: "His remark about resistance to change is likely to anger many heads"

But Mr Blair told teachers they must take a lead in modernising education and insisting on the highest standards.

The chief inspector of schools, Chris Woodhead, backed the criticism from the top of government.

Nigel de Gruchy: Blair is blaming teachers and doctors for his own failures

"He is criticising those elements within the profession, those who purport to speak on behalf of the profession, who have resisted changes over the last decade or so - the unions, the local education authorities, elements within teacher training institutions, academia," Mr Woodhead said.

But the Education Secretary, David Blunkett, sought to soothe irate teachers. "We are not knocking teachers," he said.

He added: "Every time we take a step forward there are those who resist it.

David Blunkett: "We are not knocking teachers"

"Once it has been established it is welcomed and encouraged. We have to take the next steps forward."

Mr Blair reiterated this theme in a speech to new headteachers on Thursday, highlighting the success of literacy and numeracy strategies in England's primary schools.

"What have we got do is get the money and investment in but tie it to the change and modernisation. Make sure we are removing the bad and rewarding the good," he said.

Free laptops and criticism for teachers

He also announced the opening of a new National College of School Leadership in Nottingham.

Every school head will be given a laptop computer linked to the new college, to enable them to share ideas and good practice, he said.

But Mr Blair claimed too many teachers have little ambition, reject excellence and treat poverty as an excuse for failure.

'Forces of conservatism'

The BBC's Niall Dickson: "Labour had promised great things"

The prime minister first attacked the "forces of conservatism" in his speech to Labour's party conference last month.

It provoked an angry backlash from many right-wing politicians and commentators, particularly over a passage in which Mr Blair suggested these forces had led to the murders of Martin Luther King and Stephen Lawrence.

But while it was suggested the prime minister had backed away from the confrontational stance, he now appears to be making it central to his public image.