Peers could force a new clause into the higher education bill to ensure that no future government could claw back extra top-up fee income by cutting public funding for universities.
A widely supported amendment from four leading peers would guarantee that state funding levels would be protected in future and that all fee income would "in principle" be additional to and not in place of state funding.
The amendment was tabled by crossbencher Lord Dearing, author of the landmark 1997 report on higher education and chancellor of Nottingham University; Liberal Democrat Lord Phillips of Sudbury, chancellor of Essex University; Labour peer Lord Puttnam, chancellor of Sunderland University; and Conservative Front Bench Spokesman Lord Forsyth.
Introducing the amendment at the bill's committee stage, which continued this week, Lord Phillips said that although ministers had given assurances that the fee income would be additional, it would be a "deep-dyed farce" if such a fundamental issue rested on "mere ministerial assurance".
Lord Baker, former Tory Education Secretary, said that "such pledges were written in sand" and needed statutory guarantees.
Lord Dearing said: "I suspect that we all want a safeguard against the possibility that the increased funding will be snitched by the chancellor."
The government clawed back the equivalent of fee income after flat-rate fees were introduced in 1998.
Baroness Warwick, chief executive of Universities UK, supported the amendment despite an earlier UUK briefing on the bill, which said: "We accept that it is very difficult to commit future governments to levels of expenditure."
Labour peer Lord Winston, the Imperial College London fertility expert, and the Liberal Democrat front bench also indicated they supported the amendment.
Baroness Ashton, the government's education spokeswoman in the Lords, said the government was committed to increasing public funding. But she said she could not commit the government and future governments "for all time".
Her offer to "consider further" what we might do on the issue did not impress Lord Phillips, who said the government was "conceding nothing".
Lord Phillips has called a meeting of the supporters of his amendment to plan for the report stage, where it is set to be reintroduced for a vote.
Meanwhile, a "wrecking" amendment from the Liberal Democrats, designed to scrap fees while leaving in place provisions to monitor university access, got short shrift.
The Tories, who officially oppose fees, declined to support the amendment on the grounds that it would leave in place access agreements that they also oppose.
The government has accepted an amendment from Baroness Warwick that will ensure the Office for Fair Access cannot interfere with university admissions.
And in a further concession, ministers have put forward their own amendments to ensure Parliament has full scrutiny of a number of powers due to be given to Offa through secondary legislation.