The prime minister has told headteachers worried about meeting his tough targets for schools that the government's education "crusade" must succeed - for the sake of their pupils and of the country as a whole.

Tony Blair told the National Association of Head Teachers (NAHT) annual conference in Cardiff that there could be more money for education after the next election - provided the government's reforms of the teaching profession went through.

Mr Blair digressed from his prepared speech to talk about people he knew in his own constituency who had natural ability but were stuck in "dead end jobs" - if they had work at all - through lack of education.

Headteachers gave the prime minister a 30 second standing ovation

Britain could not afford such unexploited talent, he said. It had to have the best education system in the world to compete internationally.

As part of that, he wanted to elevate the status of the teaching profession to that of medicine or law - "natural choices for our most able and ambitious graduates".

On Tuesday, the NAHT had published a survey which indicated that half of headteachers felt they were unlikely to meet the government's targets for raising standards of literacy and numeracy in primary schools.

Mr Blair said he was "well aware" of the demands the targets made on headteachers. He said they put pressure too on himself and the education secretary. But tackling "a culture of low aspirations and standards that pervades too much of the system" was fundamental to the country's future, he said.

"There is no greater social injustice than to give a child a poor education," Tony Blair told the conference

But he said there had to be "something for something" - visible results for the £1bn extra on offer for teachers' pay over the next two years, for teachers who choose to go onto a higher pay scale linked to annual appraisals.

He said there was "serious money" on offer but there had to be "serious reform" in return. Only if that happened would voters see that it was possible to improve things and - if there were a re-elected Labour government - there would be a strong case for education getting even more money.

"We simply cannot justify such an exceptional investment unless it is tied to a significant return," he said. "It has go to be used in some way to lever up standards and performance and there really would not be public consent for it otherwise."

The NAHT's General Secretary, David Hart, said afterwards that he found it highly significant that Mr Blair had spoken of the appraisal process involving "an assessment of the progress made by pupils" - in other words, not simply exam and test results.

David Willetts: "Government directives make photocopier the centre of school life"

Mr Hart said he thought that dialogue with the Department for Education on this point could overcome the opposition of the teachers' organisations to "payment by results".

But a Downing street source said Mr Hart should not read too much into this - ministers had always made plain they were not talking about a crude system linking pay to pupils' results.

Mr Blair, in his 45-minute speech, also drew applause from the assembly for an attack on education authorities which failed to pass onto schools as much money as possible.

"The prime responsibility for improving schools lies with schools themselves - which is why LEAs should get as much money as possible out of central bureaucracy and into your schools," he said. As things stood there was a "simply inexplicable" disparity in funding between schools in neighbouring authorities, he said.

The Shadow Education Secretary, David Willetts, said the prime minister should listen to the concerns of headteachers.

"Mr Blair's government sends out instructions to schools and local education authorities on an almost daily basis. They are running our children's education like a correspondence course.

"Heads and their staff are so busy reading the latest instructions that they are not finding time to actually do some teaching. The prime minister's directives have made the photocopier the centre of school life."