More than a quarter of secondaries that control their own admissions fail to give priority to looked-after children, even though official guidance now encourages them to do so.
Changes to the admission code of practice in 2004 have brought a dramatic rise in the number of schools giving priority to children in care from one in 25 schools in 2001 to 85 per cent last year, the study found.
Academics at the London School of Economics and Political Science examined the admissions criteria of 374 comprehensives in the project funded by the Greater London Authority. They found children in care were the third most common group to be given priority in admissions after siblings of existing pupils and children living close to the school.
But while 95 per cent of community and voluntary controlled schools gave priority to looked after children, just 74 per cent of voluntary aided and 72 per cent of foundation schools did so.
A Department for Education and Skills spokesman said: "We are glad to see that schools are giving greater priority to some of the most disadvantaged children - such as children in care. To ensure that this good practice becomes universal, we have now made regulations that will require that looked-after children have top priority for places in all maintained schools."
The Government took action following a report from its social exclusion unit which said that looked-after children spent too long away from class because of instability in their personal lives and behaviour problems at school. Campaigners have warned that those who move school frequently will continue to face problems winning places at popular schools as they are likely to be full after the normal admission process is completed.
White paper 12, Analysis 18 Secondary School Admissions in London by Hazel Pennell, Anne West and Audrey Hind is at www.lse.ac.ukcollectionsCERresearch.htm