Dyslexia slows a driver's reaction time as much as moderate drinking, a report claims.
Former F1 champion Sir Jackie Stewart has dyslexia
A small Norwegian study on 17 volunteers found dyslexic drivers took 30% longer to react than other drivers.
Driving over the UK alcohol limit slows reaction times by 10%, says New Scientist.
Charities said the report was alarmist and offensive, and that having quick reactions did not automatically make a good driver.
A spokeswoman from the British Dyslexia Association said: "I would question the fact that quicker reaction times make you a better driver. I'm alarmed about that.
"Younger drivers have quicker reaction times but they have a higher incidence of accidents, which would disprove the theory.
"If someone with dyslexia has acquired a driving licence they have undergone the same tests and passed as anybody else."
Dr John Rack, of the Dyslexia Institute, said: "It's a small study and it's an overgeneralisation and oversimplification of what dyslexia is.
"It could really be quite offensive, I feel, to the many, many dyslexic people who are actually quite talented and skilled in those areas."
He pointed out that the three-times world motor racing champion Sir Jackie Stewart is dyslexic and that his driving skills are beyond question.
Dyslexia Scotland, the organisation that Sir Jackie is president of, said: "The UK Driving Test includes a hazard test during which learner drivers, including dyslexic learner drivers, have to react to a wide range of potential hazards in all sorts of driving conditions.
"If they do not pass this test, they will not be issued with a full UK driving licence."
In the study, which used a driving simulator to test reaction times, six of the 17 volunteers were dyslexic.
The researchers from the Norwegian University of Science and Technology in Trondheim asked the volunteers to complete two driving tasks.
The first involved a four-minute drive along a simulated country road at 50-80 kilometres per hour (about 30-50 miles per hour).
In the second task, the volunteers drove through a simulated city at lower speeds for 10 minutes.
The dyslexic drivers took, on average, 0.13 seconds longer to react during the rural drive and 0.19 longer to react during the urban drive compared with the other drivers.
The researchers said dyslexic drivers might have problems perceiving rapid changes in their environment such as responding to a sign when driving.