English grammar schools have been warned not to discriminate against disabled children sitting the 11-plus.
It comes after a legal challenge from a visually-impaired child who was told the day before the entry exam at Reading School he could not sit it because adjustments could not be made.
The First-tier Tribunal ruled the grammar school was responsible for putting the adjustments in place.
Reading School said it would amend its processes in line with the ruling.
The child was due to sit the 11-plus for Reading School at another school, but was told the day before by the test centre the adjustments, including larger fonts on the exam paper, could not be made because it would cost more than £2,000.
The mum of the boy, who cannot be named for legal reasons, said he was "upset and in tears" when he heard he was unable to sit the exam.
Sight loss charity the Royal National Institute of Blind People (RNIB) referred the case to the Equality and Human Rights Commission (EHRC).
At the hearing, Reading School accepted the adjustments requested by the child were reasonable but argued they had no knowledge of the request as it was made via Slough Consortium of Grammar Schools, despite allowing the alternative test arrangement to take place.
The court said consortiums should have a written policy to set out how adjustments should be made for disabled children to take the exam, according to the EHRC.
The UK's human rights watchdog said it had sent letters to all grammar schools and consortiums to remind them of their duty under the Equality Act 2010 not to discriminate against disabled children.
EHRC chief executive Rebecca Hilsenrath said: "Grammar school education needs to be available to all children and that means grammar schools making reasonable adjustments for entry exams in accordance with the law."
In a statement Reading School said it was "committed to making reasonable adjustments for all candidates relating to our entrance test".
Dr Mark Fenton, chief executive of the Grammar School Heads Association, said: "Grammar schools take their duties under the Equalities Act very seriously and make a large number of adjustments every year for children with a range of disabilities who are taking admissions tests.
"When a test is taken in a different school from the one to which the child has applied it is obviously important that the right information is shared so that appropriate adjustments can be made."