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 yesterday, Blair attacked Iain Duncan Smith's "gross insult" to teachers and students.

Attacking the government for its handling of the A Level regrading row, Duncan Smith sought to pin the blame on ministers.

"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," he said.

Blair dismissed the attack as "totally irresponsible".

The Tory leader's comments allowed ministers to mount a counter-offensive.

Education secretary Estelle Morris wrote to Duncan Smith saying that A Level students would be horrified by his "outrageous" statement.

But shadow education secretary, Damian Green, hit back, saying that both Morris and the prime minister "should be apologising to all those who have suffered because of your incompetence, and you should do the decent thing and resign".

Peter Riddell of the Times says that Duncan Smith "needs quickly to reverse the impression among Tory MPs that he is an unlucky leader."

Meanwhile the Times sketch says: "Iain Duncan Smith did not merely put his foot in it. He put his foot in it, then shot himself in the foot, then shot the other foot, and then put both feet in his mouth."