British researchers believe they have identified an underlying cause of dyslexia - a poor sense of rhythm.

As many as one child in 20 is dyslexic. According to Usha Goswami of the Institute of Child Health in London and colleagues reporting in the Proceedings of the National Academy of Sciences, infants use speech rhythms to discriminate between syllables and detect the next vowel sounds.

Scientists tested two kinds of sounds - the children were told they were Tigger and Eeyore on a toy swing and Winnie-the-Pooh on a slide - on dyslexic children and normal readers.

They found dyslexic children had trouble recognising the rhythmic sound of Tigger and Eeyore on the swing. The research arose from a discovery that dyslexic children had difficulty with rhyme which required identifying the onset of the vowel, for example the difference between fit and fat.

The experiments showed that dyslexic children seemed not to notice the "beat" that signalled the onset of a vowel sound in a sentence.

Scientists have wrestled with the challenge of dyslexia for decades. A group in the US claimed that it was genetic. Another team of London researchers showed that the English language itself was part of the problem

"This approach is now being studied in 10 languages," said Dr Goswami. "It could have significant implications for how we detect and treat dyslexia in future."