One in five state-school students failed to achieve their conditional offer last summer, Cambridge University said this week.
The failure rate partly explains why Cambridge was unable to turn a 1 per cent increase in offers to state-school students into acceptances for entry in 2003.
Instead Cambridge, like Oxford University, has seen a drop in the percentage of state- school students it takes compared with independent school students.
For Cambridge, this percentage fell from 56 per cent to 55 per cent in 2003, according to figures announced last week. For Oxford, the percentage fell from 54 per cent to 51 per cent.
Geoff Parks, director of admissions for the Cambridge colleges, said: "Just a few state-school students deferring entry or not meeting offers can easily turn a percentage increase in offers into a percentage fall in acceptances."
But he added that the failure of state-school students to achieve conditional offers made predicting acceptance rates difficult. For 2003 entry, 363 state-school students out of 1,913 failed to make their offers - a rate of 19 per cent. For independent school students 152 of 1,363 failed to make their offers - a rate of 11 per cent.
Cambridge generally expects students to get three As at A level but will sometimes ask for two As and one B grade. In rare circumstances, where a student comes from a particularly deprived background but is considered very able, a lower offer will be made.
The figures revealed wide discrepancies among colleges in terms of the balance of state and independent students accepted. King's accepts about three-quarters of its students from state schools.
By contrast, just 40 per cent of students at Gonville and Caius came from the state sector.
Dr Parks said: "Colleges are working hard to build their applications from the state sector. King's has a reputation for taking state-school students, and over 80 per cent of its applications are from that sector."
He said that, nationally, two-thirds of students with three As came from the state sector. "This is not yet reflected in our applications," Dr Parks said.