Muslim families sending children to Catholic schools
More than 26,000 Muslim pupils are enrolled in Catholic schools in England and Wales.
For the first time an annual census of Catholic schools has collected information on the number of pupils from other religions.
The biggest group of non-Catholic pupils are from other Christian denominations - but almost a tenth are from Muslim families.
The government has plans to encourage more Catholic free schools to open.
This analysis shows that, overall, nearly a third of the more than 850,000 pupils within the Catholic school system are not Catholic - a total of almost 290,000.
This can reflect local demographic changes and migration - with Catholic schools serving areas with a declining number of Catholic families.
The Muslim pupils are the biggest non-Christian group, apart from the 63,000 who are from non-religious families.
Finnuala Nelis, head of St Patrick's Catholic Voluntary Academy in Sheffield, is in charge of a school where about half the pupils are not Catholics.
She says there have been changes in the local population - and that the school is now popular with parents who choose a Catholic school, even though they are not Catholics.
This includes Christians from a number of African churches and also Muslim pupils.
She says there are Muslim pupils who regularly attend their local mosques, but who will go to Catholic religious services at school.
Talking to parents
Muslim parents are able to withdraw their children from religious celebrations at the school, says Mrs Nelis, but they want their children to participate.
"It's not an uncomfortable zone" to talk about these subjects with children or parents, she says.
Muslim parents are also able to take their children out of school on Muslim festivals, such as Eid.
She says that non-Catholic parents choose the school because of the ethos and "value system", as well as Catholic schools' reputation for a "good standard of education".
The government wants to change the rules for free schools to encourage more Catholic ones to open.
At present, free schools run by faith groups can only allocate half of their places on grounds of religion - a limit that the government has announced it will remove.
The government argues that Catholic schools combine ethnic diversity with high standards. In Catholic primary schools, 37% of pupils are from ethnic minorities, higher than the national average.
But opponents have warned that expanding faith schools will encourage social segregation.
The appeal of Catholic schools to Muslim families might also reflect the fact that there are relatively few Muslim schools in state system.
Among more than 6,800 faith schools in the state school system, only 28 are Muslim, with two more in the pipeline.
Paul Barber, director of the Catholic Education Service, said Catholic schools were "beacons of diversity and integration up and down the country".
"Often, parents of different faiths and none value the distinctive and unapologetically Catholic ethos of the Church's schools."