Study goes against the instinct of politicians and the public, researchers say.
Students who score a B grade in A-level maths today would only have received an E grade 50 years ago, new research suggests.
But despite standards dropping in the past half-century, there is no evidence that they have fallen since the 1990s, according to a study by academics at Loughborough University.
Major reforms to exams in England are currently being introduced, with the first new GCSEs and A levels in subjects including English and maths brought in last autumn.
Ministers have previously said that changes to the system are needed to make the qualifications more rigorous.
The study involved A-level maths papers from the 1960s, 1990s and 2010s at grade A, B and E. Maths experts looked at pairs of papers and decided which one showed the better mathematician.
The researchers concluded that a grade B in a maths paper from the 2010s (which are now being replaced by the new qualifications) was equivalent to an E in the 1960s, but no different from the 1990s.
Study author Ian Jones said: "In terms of the evidence, what we've got is no evidence of a decline in standards since the 1990s. What we do have is evidence of is a decline in standards since the 1960s."
He added: "The lack of change since the 1990s was something that we did not expect – that's not the intuition of politicians and the public."
A Department for Education spokesman said: "We have introduced a new, more rigorous maths curriculum at GCSE and a gold standard A level. The changes we have made will help to tackle the grade inflation of the past."