The proportion of university students awarded first-class honours has rocketed by 80% in the last seven years, prompting renewed calls from government for an immediate clampdown on grade inflation.

New figures released by the universities regulator, the Office for Students (OfS), show that 16% of students who graduated from an English university in 2010–11 attained the highest possible degree classification, but by last summer the proportion had gone up to 29%. Over the last year alone, the proportion of firsts has increased by more than 2%.

The surge in first-class degrees is not confined to a handful of rogue universities. Ninety-four percent of the 148 higher education providers included in the OfS analysis demonstrated a “statistically significant” but unexplained increase in the proportion of first-class degrees awarded last summer compared with 2010–11.

Among them, the University of Surrey increased its proportion from 23% in 2010–11 to 45% last summer; Huddersfield rose from 15% to more than 40%; Staffordshire jumped from 14% to 34%; and Greenwich from 15.5% to 37%.

Damian Hinds, the education secretary, described the figures as “disappointing” and warned that they risked undermining public trust in standards at English universities. “It cannot be right that some students are awarded higher grades for the same level of achievement than those from previous years.

“We owe it to the hardworking students and institutions who play by the rules to stamp out this unfair practice. I expect the OfS to challenge those providers where the data suggests there are unjustified and unexplained increases in attainment, and demand evidence of the actions they are taking to stop this.”

The OfS conducted a similar exercise last December and at that time issued a stark warning to universities that they could be fined or even removed from the official register if they failed to tackle spiralling grade inflation.

Higher education staff suffer 'epidemic' of poor mental health Read more

In the past, the sector has countered accusations of grade inflation, arguing that higher education has changed in recent years, with universities putting more emphasis on the quality of teaching, and students paying high tuition fees working harder in order to get the best outcome from their investment.

According to the OfS, however, even taking into account factors like entrance qualifications and student characteristics, which may influence attainment, there is still an “unexplained” difference of 13.9 percentage points compared with OfS predictions for the share of firsts in 2017–18.

The regulator acknowledged that there were positive signs that the higher education sector had begun to tackle the issue, and said it would take time for change to take effect.

Susan Lapworth, director of competition and registration at the Office for Students, warned however: “Worries about grade inflation threaten to devalue a university education in the eyes of employers and potential students. So it is essential we regain and maintain public confidence in the reliability of degree classification.”

The OfS is now planning to contact universities with the highest unexplained increases in degree classifications. “We will ask them to provide further information to help us understand how they account for these increases.” said Lapworth. “In seeking this additional information, we recognise that there are factors that could explain the increases – for example, improvements in learning and teaching – that we have not been able to measure in our analysis.”

Chris Hale, director of policy at Universities UK, the umbrella organisation that represents universities, said the sector was committed to protecting its world-leading reputation. “Students deserve to have qualifications which they can take pride in, and employers and the wider public need to have confidence in the results students achieve.

“This commitment was shown by the overwhelming support for the recent sector-wide statement of intent which maps out a framework for action, and we know that institutions are already taking action to review their processes and data.

“It is important not to forget that students working harder, improvements in teaching, investment in academic support and widening participation initiatives are also all leading to legitimate grade improvement, and such improvements are to be celebrated.”