Lawyers acting for a 12-year-old Muslim girl have condemned as "irrational" her school’s decision to ban her from wearing a full face veil, arguing that the child’s three sisters were all allowed to do so.

The High Court heard that the girl’s three elder sisters had all attended the same school in the past — two of them under the present headteacher — and had all worn the niqab with no objections from teachers.

The three young women had participated fully in activities and the school was "very supportive of them as devout Muslims and the way they expressed their faith", said Dan Squires, counsel for their sister.

The decision not to allow the youngest girl to wear the niqab was against the principles of rationality, he told Mr Justice Silber.

The girl, referred to only as X — her identity is protected by an anonymity order as is her Buckinghamshire school’s — also argued through her lawyers that the ban went against her "legitimate expectation" that she would be allowed to wear the niqab, and breached her right to freedom of "thought, conscience and religion" under Article 9 of the European Convention on Human Rights.

She began wearing the niqab last September upon reaching puberty but was told by the school at the start of term that it was not acceptable because teachers believed it would make communication and learning difficult.

The school — a girls’ grammar with an outstanding reputation — has about 1,300 pupils, including 120 Muslims of whom up to 60 wear the hijab headscarf. It is thought that X is the only pupil demanding the right to wear the niqab: a full veil which covers all of the face except the eyes.

Last year the Law Lords ruled that a Luton school was justified in barring Muslim schoolgirl Shabina Begum from wearing a jilbab, a long loose gown, to classes.

The Muslim Council of Britain has said the policy of allowing the hijab headscarf is "quite sufficient to meet Islamic requirements".

The school contesting today’s case has won the backing of The Muslim Educational Centre of Oxford, which stresses that not all Muslims agree with the wearing of the niqab.

Mr Squires told the judge that the school’s written policy on uniform over the years made no mention of Muslim dress.

This was because there was an unwritten policy based on "custom and practice" regarding additional clothing which could be worn.

There was no indication that the policy on the niqab would change until the headteacher saw X standing in a lunch queue in early September last year.

According to X’s father, 26 of the teachers at the school had been there when two of his elder daughters were pupils.

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