Sue Kershaw, as one of 11 children, said that education in her family was never much of a priority. So when she found herself struggling to spell words and understand dense textbooks while at school in the 1960s, she was left to ‘figure it out’ alone.

The now 73-year-old says: ‘I smiled and winged it – but underneath I was frustrated and lacked confidence.’

Mother-of-one Sue, from Suffolk, left school at 15. For decades she made do, working in retail and starting a family.

But acting on an impulse in her 40s to go to college and study for GCSEs, she made an astonishing discovery.

‘For the first time, I had a teacher who was remarkably patient and understanding,’ she says. ‘We were trying to read Thomas Hardy for the English Literature exam, and she could see that I was struggling. She picked up that there was something more going on, and suggested that maybe I had dyslexia.’

Sue had heard of the common learning disability that makes reading, writing and spelling such a struggle, but she had never considered she might have it.

As she researched the condition, she found the symptoms matched up exactly with her own experiences. In 2012, as a 60th birthday present to herself, Sue paid to see a dyslexia specialist.

The psychologist confirmed the teacher’s suspicion. The condition cannot be cured but Sue insists that getting the diagnosis has changed her life.

Jamie Oliver’s recent documentary, Dyslexia Revolution, about his experience of growing up with undiagnosed dyslexia, also revealed that he did not read a book until he was 33

‘I’m not the same person I was before,’ says Sue. ‘When I was younger, I was never confident because I was embarrassed that reading and writing didn’t come as easily to me as other children.

‘After I left school, I just stopped trying to read books – I thought it wasn’t for me. Now I know why I struggled, and that I can read but I just take longer and have to work harder at it than other people.’

Sue’s struggles are far from unique. Jamie Oliver’s recent documentary, Dyslexia Revolution, about his experience of growing up with undiagnosed dyslexia, also revealed that he did not read a book until he was 33.

He said: ‘I knew I wanted to be a chef, so I had somewhere to go, catering school, where I could thrive. But not everyone is as lucky as me.’

The celebrity chef has now called on the Government to test all children so that those who are found to have the condition can be offered extra support.

Two weeks ago, Dr Ellie Cannon wrote about how she shared the concerns of Oliver, now 50, about the number of children going undiagnosed. The Mail on Sunday GP columnist asked if any readers had found out later in life they had dyslexia – and she was flooded with emails and letters.

There are 6.3 million people in Britain with dyslexia – making it the most common learning difficulty – yet studies suggest four out of five cases go undiagnosed at school, meaning there could be millions of adults who do not realise they have the condition.

Dyslexia impacts how the brain processes information – particularly the sounds within words, which are crucial for reading and spelling. And while those with dyslexia are usually just as intelligent as their peers, it can cause long-lasting problems.

Part of the issue is that tests for the condition are rarely available on the NHS, both for children and adults.

Formal assessments, by either an educational psychologist or qualified specialist teacher, typically cost between £500 and £1,000. This is why adults such as Sue have slipped through the net.

Kay Carter, chief executive of charity and diagnostic centre The Dyslexia Association, says: ‘We see lots of adults coming to us for diagnosis and help, likely because the condition wasn’t well recognised when they were at school.

‘Often adults seek help when certain things happen – going back to university, doing a course at work, or a line manager changing. And for many it’s a bit of a quest for self-awareness. They’re finally asking, “Maybe I wasn’t stupid.” ’

Evelyn Atkins, now 75, did not realise she had dyslexia until she was on a management training course at work – when she was 47.

During a test, she and the colleague beside her noticed they were having the same difficulties. Evelyn says: ‘He asked me if I had dyslexia, like him. I booked a test and, lo and behold, I did have.

‘I had always managed to get around things by using little tricks. But I was really bad at speaking up for myself because I didn’t think I had the right vocabulary. Now I feel much more confident because I understand it’s not my fault I struggle with words.’

An early dyslexia diagnosis can help improve children’s chances at school. They can be given extra reading and spelling lessons plus additional time in exams. While adults will not benefit from these measures, specialists claim a dyslexia diagnosis can still improve lives.

Sue Kershaw was diagnosed with dyslexia on her 60th birthday, and while the condition cannot be cured, she insists that getting the diagnosis has changed her life

One reason is that the disability is protected under the Equality Act. Workplaces must provide ‘reasonable adjustments’ for such employees. The law also means companies cannot use dyslexia as an excuse not to hire someone.

Educational psychologist Dr Sally Eccleston says: ‘Dyslexia patients are often eligible for extra support at work. Depending on the severity of

their difficulties, this might mean they are expected to take on less written work or they might be given visual templates – diagrams explaining tasks rather than written instructions.’

She adds: ‘Often the main benefit of a diagnosis is that it boosts self-confidence. There are people who have gone decades questioning their abilities. When they find out that reading and writing difficulties aren’t their fault, this can really improve wellbeing.’

While symptoms vary between patients, there are some behaviours that may suggest someone has undiagnosed dyslexia.

Dr Eccleston says: ‘Fatigue from reading, having professional success but struggling to produce error-free work, having difficulty organising tasks or recalling information, are common issues.

‘The key is that these problems are persistent. Many people have “off” days, where they feel they are struggling to read fast or are making spelling mistakes, but for those with dyslexia these problems are permanent.’