A radical proposal by the former education minister Andrew Adonis for a new breed of Oxbridge colleges designed to improve access for under-represented students has been greeted with little enthusiasm by the two universities.

Under the plan Oxford and Cambridge universities could speed up their efforts to widen participation by opening new colleges focusing exclusively on “access with excellence”, according to Adonis, the man credited as being the architect of Tony Blair’s education reforms.

His scheme, outlined in an article published in the Guardian, would see the colleges recruit undergraduates from what he called the “missing 3,000” UK schools that supply few or no successful Oxbridge applicants.

But the two universities said they would prefer to concentrate on their existing long-term efforts to widen participation, which they argued have been an unheralded success in improving the numbers of state school and ethnic minority students enrolling.

Adonis likened the new colleges to those created for women in the 19th century, including Somerville College, Oxford, and Girton College, Cambridge, and halls for religious dissenters outside the Church of England, such as Mansfield College, Oxford.

“These colleges would significantly increase the number of places available for students from schools and families without an Oxbridge tradition. Over time, they might also transform the culture of the two universities,” Adonis said.

“Oxbridge vice-chancellors and admissions tutors prattle on about ‘widening access’. But it doesn’t happen, beyond a marginal shift over decades, driven by the constant focus on statistics on the overall private and state school share of places, which favours teenagers from the remaining 160 state grammar schools and the most successful state schools.”

But senior figures at both universities fear that creating colleges with a remit to improve access could provoke accusations of “ghettoisation” and social division.

Prof Martin Williams, Oxford’s pro-vice-chancellor for education, said: “We know that our undergraduates value the chance to mix with and learn from fellow students of all backgrounds, including our international students.

“Oxford colleges were once segregated on the basis of gender and we don’t want to create new divisions on any grounds. We share Lord Adonis’s aspiration to ensure the opportunities of an Oxford education are open to all talented students but his plan does not offer the across-the-board change we are looking for.”

The university has taken radical steps in recent years, including the launch of a foundation year for talented but disadvantaged young people at Lady Margaret Hall, Oxford, led by the former Guardian editor Alan Rusbridger, while a further group of colleges are looking at starting a joint foundation year programme.

Prof Williams said Oxford worked “incredibly hard” in its access efforts, contacting and visiting more than 3,000 schools across the UK each year.

“We know this works – we have just made our highest-ever level of offers to state school students, at 63% of the total. Since 2013 we have also seen a rise of nearly 4% in offers to the two most socially disadvantaged groups in the UK,” Williams said.

“That said, we know we have more work to do. We are actively developing new policies to deliver step-change in the students who consider and successfully apply for an Oxford education.”

A spokesperson for Cambridge said the university was aiming to raise £500m to fund additional access and outreach work, which would see it spend far more than any other UK university.

“Cambridge is making consistent progress in reaching students from under-represented groups. In the 2017 intake, we admitted more state school pupils than ever previously recorded,” the university said.

“We know we can do better still but we cannot do it in isolation. As a country, we must focus on raising ambitions and attainment levels in schools and on changing perceptions among parents and teachers.”

In a similar proposal, a paper published by the Higher Education Policy Institute called for grammar schools in England to be allowed to expand into areas of disadvantage to boost the numbers of pupils from those areas entering Oxbridge and similar universities.

“Compared to other countries, we have a hyper-selective university system. Given so many people benefit from attending a grammar school, it seems what works for universities may also sometimes work for schools,” said Nick Hillman, HEPI’s director.

But the paper’s analysis was criticised as naive and misleading by academics who have studied education and social mobility.

“As the vast majority of disadvantaged children do not attend the grammar schools, the negative impact is likely to dominate any positive impact for those that do such that overall social mobility it harmed by grammar schools,” said Lindsey Macmillan of UCL and Matt Dickson of the University of Bath.