A significant minority also think the proportion of Oxbridge students from state schools is much lower than it actually is (about half), with some even putting it as low as 10 per cent.
These findings, from the first-ever independent research on Oxbridge admissions, shows that both universities still have a long way to go to shake off their exclusive image.
The survey of pupils and teachers was commissioned by Oxford and Cambridge from the National Foundation for Educational Research to find out what attracted potential students and what put them off.
It found that both teachers and pupils in state schools and colleges needed a great deal of reassurance that they were not at a disadvantage when applying to Oxford and Cambridge.
Two-thirds of the teachers and lecturers polled thought their pupils were disadvantaged by the interview procedure at the two universities. More than a third thought their pupils lost out because of the extra tests and exams imposed by Oxford and Cambridge and a fifth thought they were put at a disadvantage by the grades demanded.
But neither teachers nor pupils thought the earlier application date or the extra application form discouraged applicants.
The survey also found that state school teachers were more likely to encourage their bright boys to apply to Oxbridge than they were their bright girls.
The most common reason for not intending to apply to Oxford or Cambridge was a belief they would not achieve the necessary grades. Other reasons included lack of courses, a perception that Oxford and Cambridge were "elitist" and the academic pressure.
The findings are to be released today at a conference on Oxbridge admissions at Wembley Conference Centre organised by Lifetime Careers.
In an article in today's TES (see page 16) Professor Sir Alec Broers, vice-chancellor of Cambridge University, emphasises that the university wants to attract bright women and men from all backgrounds and declares:
"The only elitism acceptable at Cambridge is academic excellence."
Sir Alec says he remains convinced that the interview process helps rather than hinders the university in drawing students from a wider background because the interviewers can spot potential.