A-level maths standards down on 1960s but not on 1990s
Students who achieve a B in A-level maths today would only have secured an E in the 1960s, suggests research.
However standards have been stable since the 1990s, with no evidence of any further fall since then, says the Loughborough University paper.
The researchers compared the level of mathematical knowledge needed to tackle today's maths A-level papers with those from the 1960s and 1990s.
The government said its reforms would help tackle grade inflation in England.
The authors say their work, published in the British Educational Research Journal amounts to one of the most comprehensive studies into A-level standards.
No recent decline
They looked at 66 A-level scripts across the decades and concluded that a B-grade from today was the equivalent of an E in the 1960s - but unchanged since the 1990s.
Lead author, Dr Ian Jones of the university's Mathematics Education Centre, said the study had been conducted amid "ongoing concern that maths A-levels are getting easier".
But he added: "Whilst our study does show a decline in standards between the 1960s and 1990s, there is no evidence to suggest there has been further decline in the last 20 years."
Dr Jones said the researchers had not expected the lack of change in the standards since the 1990s but said the study was "robust".
"With debate continuing about the standard of maths exams, it's important the decision-makers have the best evidence available to them."
Major reforms to GCSEs and A-levels in England were brought in last autumn with the aim of making the qualifications more rigorous.
New tougher GCSE maths courses were brought in last September, along with English and English literature. Other GCSE subjects will follow suit over the next two years, while new A-level courses will be introduced for first teaching in 2017.
The government said the changes were designed to boost standards.
"We have introduced a new, more rigorous maths curriculum at GCSE and a gold standard A-level. The changes we have made will help tackle the grade inflation of the past," said a Department for Education spokesman.