Teachers' misconceptions about Oxbridge could be preventing bright state school pupils from applying for places there, an educational charity says.
54% of Oxbridge undergraduates are from state schools
A poll of 500 teachers for the Sutton Trust suggested just 54% advised their brightest pupils to apply for Oxbridge, while some 45% rarely did so.
And nine out of 10 underestimated the proportion of state pupils at Oxbridge.
Just 8% of teachers surveyed picked the correct range of between 51% and 60%. The present figure is 54%.
The majority of teachers, some three-fifths, thought 30% or fewer Oxbridge undergraduates were from state schools.
In total, some 91% of teachers underestimated the representation of state school pupils - while only 1% over-estimated it.
The chairman of the educational charity, Sir Peter Lampl, said teachers' misconceptions about Oxbridge were alarming.
"They clearly have an impact on the number of bright state school students applying to these two great universities, despite the considerable efforts that both are making to reach out to them," he said.
But he added: "It is clear that much more needs to be done to dispel the myths about Oxbridge and other leading universities, and to ensure that young people's higher education decisions are based on fact not fiction."
The poll also suggested the majority of teachers think it is more expensive for undergraduates to study at Oxbridge.
The two universities charge the same tuition fees as others in England and offer generous bursaries for students from poorer families.
The director of undergraduate admissions at Oxford University, Mike Nicholson, said the role of teachers in career guidance and advising students on university choice should not be underestimated.
"This is why we have substantially increased our resources regarding schools liaison and widening participation," he said.
"This allows us to attend more teachers' conferences across the country and we are also, for the first time, running a joint teachers' conference with Oxford Brookes University in February 2008."
He also said that outreach activities such as open days and joint regional conferences with Cambridge were also aimed at teachers.
A Cambridge University spokeswoman said the poll results chimed with the university's own anecdotal experiences.
Admissions director Geoff Parks said: "Teachers are key influencers and advisers of young people and it is vital that the advice they give is based on up-to-date and accurate information."
Cambridge worked hard to dispel these misconceptions by holding conferences and sessions with teachers, careers advisers and student mentors from state schools which do not normally apply to the university, he said.
It was also trying to establish links with local authorities in efforts to widen participation.
A spokesman for Universities UK said the research highlighted the need for school leavers considering applying to university to have accurate and timely information about higher education.