The findings compared graduates with the same A level grades who left university with the same degree.

Dr Robin Naylor and two colleagues from the University of Warwick spent three years analysing data on 118,000 students who graduated in 1993 from 57 British universities.

They compared average earnings in the jobs students were doing six months after graduating and found that - everything else being equal - a graduate who went to an independent school was likely to be in a job where earnings were 2.5% higher.

"That meant students of the same age and sex, from the same family background in the same region of the country, got the same A-level results, went to the same university, studied the same subject and came out with the same degree," said Dr Naylor.

"If one went to an independent school, they would be likely to be in a job which would pay better than another who went to a state school."

State school pupils 'do better'

Dr Naylor said he was "disappointed" at the findings, which showed that private education could still buy professional advantage.

"Tony Blair says what his government is about, and what he wants the country to be about, is education, education, education.

"This research suggests that what is actually important is education, the school you went to and parental background.

"It shows that we still have some way to go in Britain before the government's aspirations are realised."

The same research also found that state school pupils were likely to get a better degree than those who were independently educated.

Comparing students of otherwise equivalent characteristics, a student who went to a fee-paying school before going to university was one third less likely to graduate with a first-class degree, and one-fifth less likely to graduate with an upper second-class degree.