Approximately 10 per cent of the population are affected. Six per cent have mild to moderate problems and four per cent have severe difficulties.
The British Dyslexia Association says that while children have a reasonable chance of getting the right support, many adults don't.
The charity has launched a campaign called It Matters. Adults should have easy access to a free assessment, it says. There should be at least one tutor trained in dyslexia in every post-16 education institution, and employers should get dyslexia awareness training.
Carol Ortom, BDA policy manager, says: "There's not enough expertise within schools to pick up the early signs. Teacher-training courses also need to look at dyslexia in much more detail.
"Consequently, many adults are not aware they have dyslexia. Others don't have the confidence to ask for help."
A recent report by a Department for Education and Employment working group recognised that many adult dyslexics had been intensely damaged by negative school experiences and aknowledged current provision was inadequate.
The report also recommended that specialist support services should be available to all.
Carol Orton adds: "The government has expressed genuine concern about the problem but I'm not sure there's enough will or teeth to back all these things up."
Yet funding is available nationally to support dyslexic adult learners. The Further Education Funding Council awards grants of between pound;1,000 and pound;18,000 to assist students with learning difficulties, which includes dyslexics. Colleges can also apply for funding to run specialist courses.
Meanwhile, specialist teachers such as Lauraine Parkinson at Handsworth College, Birmingham, who are trying to help 40 people on their own, are desperate for extra assistance.
"The funding for dyslexic students is available but there's a shortage of trained teachers to support dyslexic adults, especially in the West Midlands," says Mrs Parkinson, who has a waiting list. "I find it staggering that I still see people in their 20s who have a reading age of six. It just seems so terribly wrong."
Bristol Dyslexia Centre 0117 973 9405; British Dyslexic Association 0118 966 2677; Adults Dyslexia Association 0207 924 9559