The Qualifications and Curriculum Authority, who set the test, insisted that lowering the pass mark did not mean that standards were lower, but rather that the examination was harder, The Sunday Times reports.
The exam results will be published next month.
The decision may lead to speculation that a shortage of mathematics teachers is causing a chronic decline in classroom standards. A report by Professor Adrian Smith, of Queen Mary University, London, warned last year that schools were unable to recruit teachers with adequate qualifications.
Coventry University, which introduced a maths test for engineering students 14 years ago, calculated that standards had declined over that period to the extent that students who failed the exam 10 years ago were probably at the same level as those who scored a B grade last year.