A defiant Tony Blair has refused to raise taxes on the rich in order to solve university funding problems and defuse the backbench rebellion over top-up fees.

Interviewed in today's Guardian, the prime minister insisted graduates must pay their fair share and warned Labour rebels that the issue was a "test for progressive politics".

Student debt has increased to £8,000 for a three-year course before top-up tuition fees have been introduced. A MORI survey published today shows that the amount owed has risen by 74 per cent in four years.

However, in better news for Downing Street, the Organisation for Economic Co-operation and Development said yesterday that the government's plans were "commendable" and praised as a role model for the rest of Europe.

Yesterday Blair continued his charm offensive to win over backbenchers, spending two hours in private talks with small groups of rebels.

Meanwhile, the medical student who put the prime minister on the defensive during a television debate on the issue earlier this week, continued to enjoy the media spotlight.

Julia Prague, 19, who is in her second year at Guy's, King's and St Thomas's School of Medicine, told reporters: "What I cannot accept is that he said to me it is fine to incur £30,000 to £40,000 in debt to train as a doctor, simply because I am going to earn more on graduation.

"I cannot accept that. It is not good for anyone to be in that sort of debt, particularly at 25 years old when you don't have a mortgage either."

The Times reports that chiefs at the Labour Party fear that education secretary Charles Clarke could lose his seat at the next general election, while backbench rebels have been in talks with Liberal Democrats over how to defeat the government on the issue.