The document reveals that it is not only independent schools that have fallen victim to government-imposed targets to broaden the social mix of undergraduates.

It shows that students from low-achieving state schools are given preference and chosen for degree courses, even though their A-level grades are significantly worse than their rivals’ at better schools.

The disclosure is likely to provoke claims that universities are being forced by the government into social engineering and propping up weaker schools by lowering standards to take their sixth-formers.

John McIntosh, headmaster of the London Oratory, the Catholic school attended by Tony Blair’s children, attacked universities for using “under-the-counter” devices to hit their targets.

“It is very confusing for parents when they are choosing a school. They are not going to send their children to poor schools, but they will care about the unfairness of these schemes. It is not up to universities to compensate for low-achieving schools,” he said.

The list of schools was circulated to admissions staff by the then principal of King’s College London, ranked 13th in the Sunday Times university league table. It has an annual intake of 3,200 students, 70% from the state sector.

It shows, for example, that a candidate from John Roan school, a comprehensive in Blackheath, south London, could win a place with a C grade and two Ds at A-level when the normal offer was three Bs. By contrast, students from Pate’s, a grammar school in Cheltenham, Gloucestershire, would receive no such allowance.

King’s is among a number of universities that have pledged to increase the intake of students from underprivileged backgrounds in return for charging top-up fees of £3,000 a year. Cambridge, Exeter, Leeds and York have all made specific numerical pledges to the university access regulator to broaden their intake.

The scheme by King’s to recruit more students from state schools is laid out in an e-mail sent in August 2002 by Arthur Lucas, then the college principal, in which he admitted it could lower the institution’s entry standards.

It outlines an initiative agreed by the college’s admissions committee to help the “many students [who] do not reach their full potential at A-level because they are relatively disadvantaged by their secondary education”.

It covered applicants going through “confirmation”, a process for those who narrowly missed the grades required by King’s, and those from the central clearing pool who had failed to get the grades.

Tutors were also given a list of 300 schools — mainly poorly performing comprehensives plus a handful of private schools — that would benefit from the scheme because their average A-level scores were below the national average. Admissions officers were allowed to give applicants notional “bonus” points that could lift them by as much as five A-level grades across their three subjects.

By contrast, tutors were told that applicants from 300 other state and 327 independent schools on the list whose results were on or above the national average should not be given any advantage.

Lucas admitted in the e-mail: “The potential disadvantage is that it will lower the college’s entry level score in newspaper league tables but I do not believe that we should allow these to influence policy.”

He made it clear that the technique “must not be applied mechanistically” and should be used in the context of a candidate’s overall application. In addition, tutors were cautioned not to admit anyone with grades lower than three Ds, even if they were from a weak school.

King’s said this weekend that the 2002 e-mail reflected government pressure to meet quotas. A spokesman added that the scheme applied only when tutors were filling places left vacant by candidates who had failed to make their grades.

The spokesman defended the university saying: “Tutors looked for applicants with potential — those who had not performed as well because they had gone to low-performing schools. It will have affected only a small number of applicants. ”

Since then, the scheme has been modified, but, according to King’s, the principle of helping candidates from weak schools has been retained. Staff now write the average A-level score of a candidate’s school on their application form. Their application is then looked on more favourably if they significantly out-perform their school’s average.

The King’s scheme, based on bonus points, is similar in principle to those tested at other leading universities, such as Bristol and Newcastle.

For King’s to achieve its current target of 76% state entrants, set by the government, it needs to increase its annual intake of state pupils by 291, at the expense of those drawn from independent schools. In an “access agreement” signed in January with Sir Martin Harris, the government’s director of fair access, King’s pledged “an early” increase on its current 70% of state school students.

King’s law department favours underachieving state schools across all of its undergraduate intake. Jane Henderson, head of admissions, said the department separated applications from schools, particularly those in London, that scored below the national average A-level results.

Experts believe the need to meet government quotas means there is a risk that some universities may now be rejecting some of the best-qualified candidates.

Elspeth Insch, head of King Edward VI Handsworth, a girls’ grammar in Birmingham, said: “My girls tell me that girls from schools up the road have been given lower offers for the same courses. But to know that is happening is disturbing.”

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