State school teachers regularly fail to advise their brightest pupils to apply to Oxbridge - with many underestimating their chances of success, a study says.
The research shows 43% rarely or never recommend that their star students apply to Cambridge or Oxford University - with 13% of those believing their pupils would not fit in.
The majority of teachers underestimate the proportion of state-educated students at the prestigious institutes, according to the poll published ahead of the Oxbridge applications deadline on Saturday.
More than a fifth of state teachers thought fewer than 20% of students come from publicly-funded schools - when the actual figure is three times that, the research for the Sutton Trust shows.
Sir Peter Lampl, the chairman of the social mobility-promoting charity, said: "Today's polling tells us that many state school teachers don't see Oxbridge as a realistic goal for their brightest pupils. The reasons are they don't think they will get in and if they get in, they don't think they will fit in.
"The majority of state school teachers believe state school students are in the minority at Oxbridge, when in fact they are around 60%. It is vital that the universities step up their outreach activities to address teachers' and students' misconceptions."
Privately-educated students are still over-represented at the universities, with around 40% of undergraduates coming from fee-paying schools - despite them accounting for less than 7% of the UK population.
The Sutton Trust says the "significant gap" has narrowed in the past 20 years and lays some of the blame with the support and advice available at state schools.
The research shows 60% of those who do not recommend Oxbridge offer no advice whatsoever on which university their pupils should apply for.
And only 21% of those polled routinely advise their top pupils to apply to Oxbridge, according to the poll of 1,607 state teachers conducted by the National Foundation for Educational Research.
A Department for Education spokeswoman said the Government is investing £90 million to improve careers education.
She added: "There should be no barrier to any child's ambitions and we are already seeing record numbers of young people going to university.
"We know the importance of good careers education and guidance in helping young people, whatever their background, make informed choices - whether that is going on to the next stage of education, training or work."