The consumer finance expert Martin Lewis has accused Ucas of abusing its position after the university admissions service allowed a private debt firm to market commercial loans with high interest rates to school leavers.

A message from Ucas to students registered with its service carried advertising for Future Finance, a Dublin-based private debt firm that offers loans of up to £40,000 to students at UK universities.

Lewis, a former BBC financial journalist who founded the consumer advice website MoneySavingExpert.com, has told Ucas that it had been “tainted” by advertising loans that were inappropriate for most students, charging interests rates far above government-backed loans.

“We have seen a number of concerns raised by shocked recipients. In allowing inclusion of this advert, we believe your charity breached an ethical line, and failed in its duty of care to the people it communicates with, which includes a high proportion of school leavers,” Lewis said in a letter to the Ucas chair, Koen Lamberts.

“Ucas has privileged, monopoly access to this young and impressionable audience. It is also seen as an institutional authority and therefore adverts contained in your email are effectively being legitimised by inclusion, and some may even mistake it for a direct recommendation.”

The emails sent by Ucas carried the introduction: “See the below information from Future Finance that we hope you find of interest.” A message below from Student Finance read: “Student finance not enough? Apply for extra funding whenever you need it.”

The message told readers: “We are backing the next generation with smart and simple finance designed around your potential, not your current situation.”

Lewis said official student loans offered by the government were “an entirely different beast from the commercial debt this email was flogging. Student loans are income-contingent – only repaid if people earn enough after university and repaid in proportion to that – and the debt wipes after 30 years.”

The commercial loans being offered have interest rates of up to 23.7%, well above the current maximum of 5.4% on student loans and worse than most high street credit cards or even bank overdrafts, according to Lewis.

“I would kindly ask that the trustees urgently consider permanently and publicly blocking the inclusion of these adverts, and any adverts from commercial debt firms, in future,” Lewis wrote to Lamberts, who is also the vice-chancellor of the University of Sheffield.

In response, Ucas said: “Ucas is an independent charity. All our activities are funded by application fees, by universities and colleges, and through gift aid from the profits of our wholly owned commercial subsidiary, Ucas Media. This helps us to keep the costs for students applying to university as low as possible.”

It said users could opt out of marketing emails at any time, and said that Ucas “always advises that the best option for students is a government-funded student loan”.

Future Finance said it always advised applicants to check they had made full use of the government support available to them before considering taking out a loan. Olga Dolchenko, its chief executive, said it had lent more than £100m to 15,000 UK university students during the last five years.

“Given undergraduate students often have no credit history or a poor rating, they are unable to access traditional forms of finance and either struggle to make ends meet or choose inappropriate options such as payday lending. We fill this gap, but never encourage students to borrow more than they can afford,” Dolchenko said.

The promotion of commercial loans by Ucas was initially criticised on social media after the email was sent at the end of last month. Eva Crossan Jory, the National Union of Students’ vice-president (welfare), said: “Our student loan system may be broken but encouraging students to take out private loans, some of which they will end up paying back double on, is beyond disappointing.”