Iain Duncan Smith has called on the prime minister to rule out university top-up fees for the duration of this parliament.

He said Blair was secretly planning to drop his pre-election pledge and hit thousands of middle income families.

"Before the last election he said 'we will not introduce top-up fees' and now the prime minister indicates absolutely clearly to students up and down this country...that he will be introducing top-up fees costing them thousands of pounds," he said.

"How can any family ever believe a word that he says?"

Dismissing the line of questioning, the prime minister said he would honour his election commitment.

"We will abide completely by the manifesto commitment that we gave," the prime minister told MPs.

"I suggest before he accuses us of taking a particular course of action that he awaits the outcome of the review."

Giving a hint that the government's review may not call for top-up fees, Blair told the Tory leader: "You may just find that some of your strictures are unjustified."