Lord Mandelson, the last Labour cabinet minister responsible for higher education, is expected on Friday to raise the bar for any Labour pre-election decision to cut tuition fees, arguing that any reform has to ensure that the current range and flow of funding into universities from all available sources is sustained.

He is due to deliver a speech to Universities UK as Labour continues to discuss whether to propose a cut in the maximum level of the tuition fee from £9,000 to £6,000.

Ed Miliband has been strongly supportive of the fee cut, placing a pressure on the shadow chancellor, Ed Balls, to find a credible way to find a long-term substitute for the consequent loss of income to universities.

The much deferred announcement is due at the end of the month, but some senior vice chancellors last month infuriated the Labour front bench by issuing an open letter opposing the tuition fee cut only days after meeting the shadow higher education minister, Liam Byrne. That led to threats about reviewing the salary packages of University vice-chancellors.

There had been suggestions that the party would defer a decision until after the general election and look instead at the short-term impact on state and university finances if a graduate tax was introduced. Some internal focus group polling for the party has suggested a simple cut from £9,000 to the £6,000 cap first proposed by Miliband in 2011 is not as popular as might be expected given that the party envisages lower fees being offset by other means of taxation.

Mandelson is unlikely to mount a full frontal assault on Friday and has recently strongly supported the leadership in articles and interviews. But he questions whether it would be better for the party to resolve the issue in government when the extremely complex long-term funding implications of changing tuition fees can be resolved. There is a good deal of work suggesting that the level of fee repayment is not going to meet the government initial estimates of long-term income, and this is already causing a shortfall in the Business Department estimates.

But Mandelson, aware no perfect solution is available, has yet to be convinced there are any signs that participation rates at university from disadvantaged backgrounds has been damaged by the rise in tuition fees. Some surveys suggest that more than half of British 16- and 17-year-olds see themselves as likely to go to university, implying that aspiration has not been dented by tuition fees. New ways have also been found to teach, including on shorter courses or for students that remain at home.

Nick Clegg speaking on LBC on Thursday said the fees system is proving to be much fairer than people predicted at the time, adding it would have saved him a lot of heartache if the system had been called a very progressive graduate tax.