The prime minister’s brother, Jo Johnson, has warned against a proposal to cut university tuition fees.

The younger, remain-supporting Johnson – who was universities minister until September when he resigned from government citing an “unresolvable tension” between his family loyalty and the national interest – argued that lowering student fees would do “grave damage” to higher education finances.

The former Tory MP, who stepped down at the election and is now chairman of the group that owns the Times Educational Supplement, said cutting fees would also be “very bad politics”.

The Conservative manifesto pledged to consider a review by former financier Philip Augar last year that recommended reducing fees from £9,250 to £7,500. The report, commissioned by Theresa May, suggested extended payments from 30 to 40 years, as well as reintroducing maintenance grants for poorer students.

Meanwhile, Labour said in its own manifesto that it would scrap tuition fees entirely, citing spiralling student debts. Fees were introduced under Tony Blair’s New Labour administration before they were trebled during the coalition government, prompting huge student protests. Government figures highlighted last year showed interest charged on student loans is forecast to rise by £4.2bn to £8.6bn a year by 2024.

Speaking on Saturday, Johnson backed the government’s pledged increase in science funding. He also said he wanted universities to remain properly funded, warning against lowering tuition fees. “I think that would do grave damage to our institutions’ financial stability and, also, I think it would be very bad politics as well but that is rather beside the point,” he told BBC Radio 4’s Today programme. “So we’ve got to continue to fund our universities successfully and build on our research excellence and I think that’s the priority for the government.”

Asked if there would be a significant impact if fees were lowered by a third, he replied: “Yes, there would be a substantial impact, particularly if that funding were not made up by the Treasury which, given the current politics, I would have grave doubts that it would be. The priority, where there is discretionary income within the Department of Education, is to put it towards schools and to put it towards further and technological education. We need to level up, rather than level down, university funding to create some sort of false parity.”

He added: “I think where there clearly are grounds for concerns about universities are, as the sector has expanded, has quality been maintained? Are there issues around degree inflation? Are there issues around unconditional offers? Which are legitimate areas for criticism and for reform but I think to be vindictive and to be, sort of, punitive about universities because they were on the wrong side of a perceived culture war over Brexit, I think that’s completely the wrong way to go if we want to make a success of global Britain and our future post-Brexit.”

Johnson also defended universities after Sir Robbie Gibb, Theresa May’s former director of communications, laid into the the Today programme’s election coverage in a comment piece for the Daily Mail. In a scathing piece, Gibb said the programme had “misread the politics of the election with endless outside broadcasts in universities, full of interviews with left-wing, entitled, virtue-signalling students”.

Responding, Johnson said: “Our universities are clearly a great national asset and if we want to make a success of Brexit projects like global Britain, we need our universities to play a very big part in that. We need to be championing them and getting behind them and making the most of them. That means funding them properly and it means we don’t run down institutions like our universities and denigrate their cultures.”