The school has been forced to act after the Appeal Court said Al Hijrah, a state-aided Muslim school in Birmingham, was breaching equality law by segregating boys and girls completely from the age of nine.
The implications of the ruling are being digested by the education authorities and by other Jewish schools where boys and girls are taught separately to varying degrees.
Ofsted believes that the number of Jewish, Christian and Muslim schools that could be affected is in the low 20s.
Hasmonean is planning to modernise its girls’ site in Mill Hill and relocate the boys from Hendon to a new building alongside, but separate, from the girls.