A reduction in tuition fees would make it harder for disadvantaged students to go to university, according to a group of funding charities.
A post-18 education commission, set up by Theresa May, is reported to be considering recommending fees be cut to £6,500 from the current £9,250.
At the moment universities are required to set aside part of their income to help students from poor or disadvantaged backgrounds enter higher education. That works out at about £860m for this year.
But six charities that help with university funding say that money would be reduced if fees are cut.
In a joint statement The Access Project, Brightside, Causeway Education, Impetus-PEF, IntoUniversity, and
upReach said: "Higher education (HE) should be a route open to all young people, irrespective of background.
"But we have a big and persistent social mobility problem in the UK - young people from disadvantaged backgrounds are half as likely to progress to HE as their peers.
"Widening participation funding exists to help close this gap and is vital to the work that we do to support young people from under-represented groups to progress and succeed in HE.
"That progress is now in doubt."
The charities also urged the government not to impose a cap on student numbers, and to increase the amount of maintenance support available to young people.
The statement comes ahead of the release of official figures that are expected to show a dip in the number of disadvantaged people going into higher education.
The charities' statement has been endorsed by the Russell Group, which represents 24 leading UK universities, ,which pointed to a record number of disadvantaged young people in post-18 education under the current funding system.
A Department for Education spokewoman said: "We know that widening participation in higher level study post 18,
at universities and further education colleges, is vital.
"That is why we are looking into this as part of our post 18 education and funding review.
The commission to review post-18 education was set up by the Prime Minister in February this year.
Apart from tuition fee levels, it considered proposals for two-year degrees, plans for which were revealed by the Universities Minister Sam Gyimah on Sunday.
The review is also looking at two-tier funding, with higher tuition fees for science degrees, while reducing those for arts and humanities.