Tony Blair has defended the system of A Levels after the Conservative leader claimed they "were not worth the paper they are written on".

In a highly charged session of prime minister's questions, Blair attacked Iain Duncan Smith after he called on the government to scrap the new post-16 qualification.

Following the recent exams fiasco, Duncan Smith said it was time to abolish the two-year-old qualification.

"He has in front of him all the evidence from parents and teachers and even from all the assessments that have been made," he said.

"So he has a very simple choice. Right now faith in A Levels has been shaken to the core, nobody even knows whether they are worth the paper they are written on.

"Why doesn't he simply accept that the AS Level is a failure and it is time to scrap it."

Blair dismissed the attack - denying that the recent Tomlinson inquiry had concluded that the exam should be abolished.

"First of all that is not what the Tomlinson report found at all," said Blair.

"Secondly, when he says that they are not worth the paper they are written on I think that is a gross insult.

"Of course this has been an immensely difficult time for those students.

"But to say that A Levels, and to the students who have got those A Levels, that they are not worth the paper they are written on is totally wrong."

Blair also told the Commons that he has confidence in the school exclusion system despite claims that head teachers can no longer control discipline in the classroom.

The prime minister said the system had worked well since it was introduced in 1987.

His comments came after Duncan Smith challenged him to say whether the government "had made a mistake" by establishing appeals panels which can over-rule the decisions of head teachers.

The number of attacks on teachers had risen four-fold since Labour took office, said the Tory leader.

"Why doesn't the prime minister simply say he has the power and scrap the appeals panel and sort this problem out," said Duncan Smith.

Blair said that Duncan Smith's attack was "factually wrong".

"It isn't true to say that this government introduced the appeals panel, because it was the previous government. And it isn't fair to say either that we haven't tightened the appeals procedure, because we have," said Blair.

The prime minister said that the appeals process had been toughened since he came to office.

"In 2001 we made it clear that exclusion panels should not normally recommend the reinstatement of pupils where there has been serious actual or threatened violence," he said.

"In addition to that there will be a teacher on every panel."