Primary teachers with a C grade at maths GCSE are 13 times less likely to understand the topics they are required to teach than those who scored A grades, new research reveals.
Ian Kilshaw, senior lecturer at the University of East London, also found that trainees who sat their GCSEs after 1999 were significantly more likely to struggle with primary-school maths than their older colleagues.
In order to be accepted on to a training course, would-be teachers must have a C grade or above in GCSE mathematics.
And all trainee primary teachers are required to demonstrate their mathematical competence during the course.
In order to determine how realistic a demand this is, Mr Kilshaw surveyed 191 trainee primary teachers, with a selection of A, B and C grades in GCSE maths.
All trainees were audited three times throughout the year to monitor their mathematical progress. They were assessed in nine areas of knowledge, including algebra, equations and probability.
The pass mark for each subject was 50 per cent, and those candidates who failed were required to resit.
Unsurprisingly, trainees who had scored an A in maths GCSE were least likely to fail their mathematical audits. During the first audit, trainees with a B grade failed in five times as many subject areas as their coursemates with A grades.
By the last audit, their performance had improved slightly, and they failed in only four times as many areas.
Meanwhile, trainees with a C grade at GCSE failed in six times as many areas as their A-grade counterparts in the first audit, and in 13 times as many in the final audit.
Mr Kilshaw said: "While trainees with a B grade catch up with trainees with an A grade across the three audits, trainees with a C grade ... fall further behind."
But trainees' performance also varied according to the year they had taken their GCSEs.
Those who had sat their maths exams after the year 2000 failed their audits in three times as many areas of knowledge than those who had taken their exams in 1999 or before.
All the candidates struggled particularly in four areas: reasoning and proof; equations, functions and graphs; shape and space; and algebra.
The Government's recent Williams Review into primary maths teaching noted that most teacher-training courses devote only 10 or 15 days to maths teaching. The report concluded: "It is fair to observe that this can be expected to bring about little change in the mathematical competence and subject knowledge of a trainee."
Mr Kilshaw suggests testing prospective teacher-trainees during their initial interview, to determine the level of maths guidance they require. And, he says, the number of taught maths lessons should also be increased.
He said: "The challenge will be to identify the areas of mathematics that need support and development, and identify each trainee's need, so that our teachers of the future have secure mathematics subject knowledge."