The number of graduates receiving top degrees in England has more than doubled in the past decade, prompting the university watchdog to threaten to clamp down on “unexplained” first-class honours.

Figures released on Thursday by the Office for Students, the university sector regulator, showed 37.9 per cent of students were awarded first-class degrees, the highest classification, in 2020-1, compared to 15.7 per cent in 2010-11.

Education experts have warned that grade inflation is leaving employers struggling to distinguish between graduate candidates and students unsure about the value of their achievements.

But university leaders said the findings did not reflect improvements in teaching or more support for students that had lifted attainment over time.

Tom Richmond, director of EDSK, an education think-tank, said the “damage to the reputation of the grading system has already been done”. “Some employers have already given up on degree classifications altogether because they no longer feel that they can trust them,” he added.

During the pandemic, universities adopted “no detriment” policies to protect grades from being adversely affected by teaching disruptions, which caused the number of students attaining top marks to jump sharply.

According to the OfS analysis, 84 per cent of students achieved a first or upper second degree in the 2020-1 academic year, up from 67 per cent ten years before.

But Susan Lapworth, interim chief executive of the OfS, said grade inflation had been “a credibility issue” in the higher education sector for some time.

“The pandemic cannot be used as an excuse to allow a decade of unexplained grade inflation to be baked into the system,” she said. “Unmerited grade inflation is bad for students, graduates and employers.”

The OfS added that six in ten first class degrees in 2021 were “unexplained”. This means they were outside students’ predicted outcomes based on their entry grades and other characteristics like sex and race.

Steve West, president of Universities UK, which represents the sector, said universities were “committed to addressing grade inflation” as well as delivering high-quality courses and improving standards.

“OfS regulations sought to make sure all students could succeed, no matter what grade they started university with,” he said. “The OfS must be careful not to assume that students with lower entry grades, typically from more disadvantaged backgrounds, cannot achieve first class degrees.”

Lapworth acknowledged that a variety of reasons, including improved teaching, could lead to an increase in the number of top degrees but said “sustained increase” was still a “regulatory concern”.

But where degrees were not “credible and reliable”, she said, the OFS was “prepared to take action” by investigating individual institutions.

The National Union of Students, which represents student bodies across the UK, said no detriment policies had been a “useful mechanism” for helping students succeed during the pandemic.