That explanation may be too simple, but most experts would agree with David Blunkett, the Secretary of State for Education, that mental arithmetic has been neglected in schools and that it may take time for pupils to acquire a skill which needs to be perfected by constant practice.
Until recently, experts have been divided on the best way to teach maths. During the Sixties and Seventies, rote learning of times-tables and mental arithmetic declined while teachers tried to instil mathematical concepts through practical activities and to introduce new topics in primary schools such as geometry.
Pupils have often been taught in small groups rather than as a whole class. There has also been a debate about the use of calculators, which are used more in British primary schools than in any other Western country.
In international comparisons, British pupils have lagged behind their peers for at least three decades. Earlier this year, a report from the Government's Numeracy Task Force, led by Professor David Reynolds, united both conservatives and progressives behind a new strategy to improve maths standards, including a daily numeracy hour.
The group backed more whole-class "interactive" teaching, in which all pupils are engaged in question-and-answer sessions, based on successful methods in countries such as Switzerland, Hungary and Taiwan. It also recommended more emphasis on number work and mental arithmetic. On calculators, it failed to pass judgement after research showed they made little difference to pupils' performance.
Improvement in maths standards, however, faces a formidable obstacle: a shortage of teachers. The recruitment crisis is affecting maths more severely than any other subject. Secondary-school maths recruitment to teacher training is 50 per cent short of its target. And there is already concern that the qualification required to train is as low as a C in maths at GCSE.