Trinity Hall graduates and leading social mobility charities have called on the University of Cambridge college to scrap its controversial efforts to actively recruit students from elite private schools, describing the new policy as damaging, offensive and a “step backwards” for equality.

After the Guardian revealed that Trinity Hall was to target a small group of wealthy private schools – so as to not “ignore or marginalise” privately educated students, according to an internal briefing – those promoting students from state schools said they were shocked and disappointed by its justifications.

Alastair Campbell, the former Downing Street director of communications under Tony Blair, told the Guardian: “For the college to talk about pupils from the top private schools being ‘ignored and marginalised’ suggests a total departure from reality, which is not a great sign for an elite academic institution.

“Being four or more times better represented in such elite institutions like Cambridge University suggests anything but marginalisation. It suggests the college wants to reverse the painfully secured advances in social mobility. And my worry would be they are doing this so that others may follow. Hence the need to raise voices against this now.”

Campbell is a supporter of the 93% Club, a network representing the UK’s state-educated students. The club’s chief executive, Sophie Pender, said its Trinity Hall members would be “withdrawing alumni-funding donations until the policy was reversed”, telling the college that its decision was “immoral and dangerous”.

Pender added: “Your state-educated alumni are watching now; please don’t set this precedent. It’s not too late to course correct, and we will respect you all the more for it.”

Under the policy adopted at the end of last year, Trinity Hall would approach independent schools to encourage applications in courses including languages, music, art history, classics and theology, to improve the “quality” of recruits. Most of the schools are in the south of England and charge upwards of £25,000 a year.

The Sutton Trust, a charity that aims to boost UK social mobility through education, called for the policy to be reversed and said it was “engaging” with the University of Cambridge on the issue.

Nick Harrison, the trust’s chief executive, said: “State-educated pupils should absolutely not be overlooked for recruitment to any university courses. We’ve previously found that those from private schools are already overrepresented on a wide range of courses, despite representing only 7% of the population. This already limits opportunities for those with similar abilities from state schools.

“Gaining a degree remains one of the surest routes to social mobility, yet pupils eligible for free school meals are only around half as likely to go to university as their better off peers. And persistent access gaps remain at the most selective institutions.”

Trinity Hall alumni contacted the Guardian and took to social media to express their dismay at the policy. Several said they had contacted the college to register objections, and in some cases that they would refuse any fundraising appeals while the policy remained in place.

One said: “This is clearly an attempt to recruit more wealthy students to boost Trinity Hall’s base of alumni with deep pockets. I am so deeply disappointed in my former college for implementing such a crude policy.”

Mary Hockaday, the master of Trinity Hall, responded to alumni on Thursday evening, telling them that the media coverage was “very misleading”.

Hockaday said in an email: “To be clear, there has been no change to our admissions policy or our commitment to widening participation, and we continue to work very hard to support and attract students from disadvantaged backgrounds.

“We propose to add some schools to our existing recruitment email lists to ensure we receive applications from talented students from all backgrounds. This activity does not affect our overall admissions policy.”

But Jess Staufenberg, a co-founder of the Private Education Policy Forum, a thinktank that studies independent schools, said there was no evidence that private schools needed any encouragement to apply to Cambridge.

“These are some of the most expensive and socially exclusive schools in the world. We know that many of these schools have entire staff teams dedicated to ensuring successful Oxbridge admissions,” Staufenberg said.

“Trinity Hall should present its evidence for why these private schools – rather than state schools in forgotten regions of the north-east, for instance – need a specific recruitment policy encouraging their students to apply.

“Trinity Hall also needs to answer why it is not using its considerable endowment funds to track down and find the talented and brilliant students hidden in the nation’s most deprived communities.”

Francis Green, a professor at University College London who has studied UK private schools and inequality, said: “If a college perceives they are receiving too few qualified applicants in particular subjects, they might do better to advertise this generally across the school spectrum, or even adjust their educational offering, rather than risk retreat on what has been one of the few positive developments in widening participation in recent years.”