The ruling was given in the case of a senior policeman who requested extra time in examinations because he said his dyslexia was a disability.
An employment tribunal ruled that he did not have a disability and so anti-discrimination legislation should not apply. It said that because the worker could function as well as other people, he should not have the protection of the law.
The EAT, though, said that that approach undermines the entire basis of anti-discrimination legislation.
The policeman, D Paterson, was 42 years old when he discovered that he was dyslexic. He had by then risen to the senior rank of chief inspector. He sought an extra time allocation of 25% when sitting exams for promotion because of his dyslexia.
Though he had performed clerical duties well throughout his career, medical reports indicated that he had difficulty with some aspects of reading and would need more time than an average person to produce the same work.
An Employment Tribunal agreed that he was significantly disadvantaged compared with his peers when sitting exams but said that since this was not a normal day to day activity, Paterson did not qualify for protection under the Disability Discrimination Act.
The EAT said that the Tribunal had made a serious mistake in its judgment. "It would wholly undermine the protection afforded by this legislation if the Tribunal were correct," wrote the EAT.
"Once it was accepted that the disability affected Mr Paterson's ability to progress in his profession, the only reasonable inference was that it had a substantial effect on his day to day activities," said the ruling. "Any other conclusion would mean that because of the effects of his dyslexia, he would in practice face a glass ceiling; he would not be able to compete adequately in the promotion stakes beyond a certain level."
The EAT also said that the way in which the original Tribunal assessed disability was flawed. "[The correct approach] is not to compare the performance of the employee with the average person in the population, which was the comparison the Tribunal seems to have made. Rather, it is the comparison between what the individual can do and would be able to do without the impairment," said the ruling.
The case will now return to the original Tribunal, but with Paterson's impairment qualifying as a disability.
Employment law specialist Ben Doherty of Pinsent Masons, the law firm behind OUT-LAW.COM, said that it is not the first time that dyslexia has been deemed a disability.
"There have been previous cases of dyslexia being accepted as a disability, however it will depend on the individual circumstances of each case and in particular the severity of the dyslexia," he said.
"Employers should consider every case of dyslexia on its own merits, it is still the case that individuals with minor dyslexia may not be disabled. In any instance of possible disability discrimination, including dyslexia, the employer should consider obtaining an expert's opinion on the employee's condition and whether it amounts to a disability. If the expert's opinion is that the employee is disabled, the employer should ensure that they do not discriminate against the individual and should consider making reasonable adjustments," said Doherty.