Inspectors have said that a strictly Orthodox girls school in Hackney should not be allowed to expand because of weaknesses in its welfare, health and safety arrangements and problems with its premises.
Bnois Jerusalem in Stamford Hill, North London, which has 780 girls on its roll from 3 to 16, applied last year to the education authorities for permission to expand to 850.
The school already takes 100 pupils more than it is officially registered for with the Department for Education.
But Ofsted inspectors, who made an unannounced visit in September, said they could not recommend an expansion "because there are weaknesses in the school's current safeguarding, welfare, health and safety provision, together with shortcomings in the premises and accommodation".
The latest report is further sign of a crackdown by the authorities on religious institutions that fail to meet independent school standards.
One independent Chasidic boys school in Stamford Hill is fighting a closure order from the DfE, while another could face a similar threat if its plans for change are not approved.
At Bnois Jerusalem, there was "overcrowding in most rooms, with insufficient room for pupils to move around within most classrooms and the narrow corridors and stairs," Ofsted said in its latest report.
"Many parts of the building are in a poor state of repair with damaged flooring and walls requiring plastering; virtually all are in need of painting. Across the school, dirt is ingrained, including in rooms where very young children play."
The inspectors also want improvements in other areas, including suitable toilet and washing facilities for pupils and better checks on whether staff are suitable to work with children.
According to an inspection two years ago, the girls, mainly from Yiddish-speaking homes, were enjoying a good quality of education.
There are 161 girls in the early years section of the school, who receive public funding.
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