The majority of undergraduates now at university will be paying off their student loans well into their 40s and 50s, with three-quarters of them unable to clear the debt before it is written off after 30 years, according to an analysis published on Thursday.

The report by the Institute for Fiscal Studies and the Sutton Trust estimates that the average student will leave university more than £44,000 in debt.

A middle-earning graduate will still owe about £39,000 at today's prices by the age of 40, and will still owe about £32,000 by 50.

"For many professionals, such as teachers, this will mean having to find up to £2,500 extra a year to service loans at a time when their children are still at school and family and mortgage costs are at their most pressing," said Conor Ryan, the Sutton Trust's director of research.

"We believe that the government needs to look again at fees, loans and teaching grants to get a fairer balance."

In cash terms, the researchers estimate that graduates will now repay a total of £66,897 on average, equating in real terms to £35,446 on average in 2014 prices.

Claire Crawford of the University of Warwick, one of the report's authors, said a perverse effect of the repayment scheme was that graduates who do less well in the labour market will end up paying back less than before, while middle and high earners will pay back much more.

"The new higher education finance system will leave graduates with much more debt than before. But the effects of the changes will be quite different for different people and at different parts of their lives," she said.

Although large numbers will repay more than they borrowed, most will not return their loan in full, the study says. "We estimate 73% will have some debt written off at the end of the repayment period, compared with 32% under the old system. The average amount written off will be substantial – about £30,000."

Liam Byrne, the shadow universities minister, said the student finance system had lost fiscal credibility. "Degree costs have trebled yet costs to the taxpayer have gone up, and now we learn our children and grandchildren will be paying off their student debt well into their 50s," he said.

The report follows forecasts that 45% of the £10bn in loans made each year will have to be written off.

The Department for Business, Innovation and Skills defended the system and said more students from disadvantaged backgrounds were going to university. "Most students will not pay up front to study. There are more loans, grants and bursaries for those from poorer families. Our universities are now well funded for the long term," a spokeswoman said.

Meanwhile, data published by the Higher Education Funding Council for England on Wednesday showed a steep decline in the number of students studying part-time at universities since the new fee regime was introduced.

The figures showed that the number of new part-time undergraduates fell by 93,000 between 2010-11 and 2012-13, while there were 23,000 fewer part-time postgraduate entrants.

The figures also showed a 22% fall in the number of undergraduates studying modern foreign languages between 2010 and 2012.

"It's important to remember the difficult choice that participating in higher education entails. Students are forced to take on a huge burden of debt," said Rachel Wenstone of the National Union of Students.