It is estimated that dyscalculia - difficulty with numbers - afflicts between 3% and 6% of the population, based on the proportion of children who have special difficulty with maths despite good performance in other subjects.

Often it is associated with dyslexia - word difficulty - but experts say the practical effects are even worse: Inability to work out change in a shop, tell the time, or even find your way around.

Dyslexia is a difference in the parts of the brain that process language. As well as problems with reading, spelling and numeracy, dyslexic people may also have organisational and short-term memory difficulties.

The chief executive of the Dyslexia Institute, Shirley Cramer, said: "For many dyslexic people the difficulties which affect their reading and spelling also cause problems with mathematics."

The two overlap because maths involves symbols as well as numbers.

For someone who has trouble distinguishing letters, a + sign might be confused with x, for example. And the language of mathematics can also be a problem, according to Bernadette McLean, academic director of the Helen Arkell Dyslexia Centre in Surrey - the oldest of its kind in the UK.

"Children latch onto the first meaning they know for words - so 'take away' is somewhere you get Chinese food on a Friday evening," she said. Finger counting And the memory shortcoming associated with dyslexia obviously caused problems with mental arithmetic.

"Even if children learn their times tables they cannot sequence backwards and forwards - so if you ask them what six times four is, then ask them what five times four is they will have to start again counting through.

"Just today I have been working with some teenage children using rows of dots. Some of them had to count them out on their fingers."

A concern with the national numeracy and literacy strategies in England was the pace of the syllabus: If a child slipped behind it became more and more difficult to catch up."You need to go at the pace that suits the learner."

Genetic link Research being done at University College, London, suggests there is a genetic basis for the problem. In other words, "I wouldn't expect my daughter to be any good at maths because I wasn't any good" might not be just a matter of poor parental attitude.

The work, by Professor Brian Butterworth, has shown that dyscalculic children are troubled by even the simplest numerical tasks such as selecting the larger of two numbers or counting the number of objects in a display.

These findings are being used to devise a new test for diagnosing dyscalculia at an early age.

This coming week, members of the British Dyslexia Association are campaigning for greater awareness of the issues involved.

One thing they want is for the colleges that train teachers to teach them more about identifying learners' difficulties and knowing how to help them.

"We want teachers going into the classroom, meeting these children for the first time, to be able to identify typical signs. That's not as good as it should be," Bernadette McLean said.