The Jewish Community Secondary School (JCoSS) in Barnet is changing its admission rules to give applicants from outside the borough a better chance of a place.
Explaining the change, due to come into effect for 2013, JCoSS head Jeremy Stowe-Lindner said: "The rationale is that we are increasingly very heavily oversubscribed.
"As the only cross-communal Jewish high school in London, we want to give applicants from the entire Jewish commuity a chance of cross-communal Jewish education.
"Because we have been so popular, the old criteria based on distance would mean that we became increasingly local."
Preference is given to Jewish pupils who can establish their Jewish credentials in several ways - synagogue membership, attending synagogue four times in the six months prior to entry, involvement in Jewish education or voluntary activity.
At present, the first places go to children from JCoSS's three feeder schools - Clore Tikva, Akiva and Clore Shalom - and siblings of current pupils. After that, places are allocated to those living nearest the school.
From 2013, priority will still go to siblings and children of the feeder schools, Mr Stowe-Lindner explained.
But then 10 per cent of places will be reserved for local children, with the rest selected through a random lottery of applicants conducted by Barnet Council.
Mr Stowe-Lindner said the religious breakdown of the students roughly reflected the synagogue membership of the different Jewish streams in London, although there was a disproportionately high number of Masorti pupils. Half of JCoSS's intake last autumn was Orthodox.