John Stein, the Oxford professor of physiology who helped uncover the evidence, said: “That’s the best thing that could come out of this. If it was clear that dyslexia was biologically based, people would not be able to go on saying it is a middle class excuse.”

Joanne Rule, chief executive of the British Dyslexia Association, said she was delighted by the findings and optimistic that they would affect attitudes. A spokesman for the Local Government Association was less sure that the news would have an impact on councils’ allocations of funds for dyslexia.

The Oxford scientists identified a section of DNA that seems to be common only to dyslexic people. They took blood samples from more than 400 people in 100 families in which two children had dyslexia. They found a link between dyslexia and a part of chromosome six, although, Professor Stein emphasises, talk of a “dyslexia gene” is premature.

“In simple language, we found that dyslexic people share a bit of this chromosome more often than they would by chance. It suggests very strongly a link between the disorder and chromosome six.”

Professor Stein also found “very circumstantial” hints that there might be a link between dyslexia and the genes that control immunity.

He said: “There is quite a lot of evidence already that dyslexics have more immune problems.” He said this could be a pointer towards discovering the cause of dyslexia.

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