An Islamic school is "undermining British values" and "limits girls to knitting and sewing" in technology classes, Ofsted inspectors say.
Rabia Girls and Boys School in Luton had "not met" standards needed for an independent school, the watchdog said.
Ofsted said the school practised "unequal treatment of girls and boys" and teaching quality was "inadequate".
The school said it was focused on "making the improvements needed to raise standards".
Rabia Girls School, based in Portland Road, was set up in 1995 as a single-sex independent private primary and secondary school in Luton.
It was the first Islamic school to open in the town and later became a joint boys and girls school due to increased demand.
But in May last year, Ofsted judged the school as "inadequate", while a follow-up visit in September 2014 said it still required improvement.
The inspection of May 2014 found pupils "were not taught a broad range of subjects in sufficient depth" and that girls "had too little career guidance", with their work experience placements limited to a nursery.
Despite some improvements, including the introduction of lessons in history and geography, the report said "the school's own work undermines the promotion of fundamental British values in its unequal treatment of girls and boys".
It said the balance between secular activities and Islamic studies differs between genders, with boys given more time to study national curriculum subjects.
The report, published on Monday, said older girls do not have the same opportunities to study science as "they do not have the same access to laboratory facilities that the boys have," while a newly introduced design and technology curriculum "limits girls to knitting and sewing".
The school currently caters for 269 five to 16 year olds.
A statement released on behalf of the Board of Governors and head teacher Mirza Akbar said the report had included "many positive comments".
It said the standards not met "were at the final stages of completion".