This is three weeks earlier than the informal half-term deadline that schools, including those in the public sector, usually use for early applications. The final closing date for applications is December 15.
The advice was sent out last week by the Headmasters' Conference and the Girls' Schools Association to member schools. It is based on a survey of university applications from 246 schools during the 1994 and 1995 applications process and represents a summary of the experiences of 19,000 candidates.
"It is clear that early application is essential in the most competitive areas and it would be advisable to submit these no later than the beginning of October," says the circular. It also lists members of the Russell Group of Universities (an informal grouping of older universities, so-called because they meet on Thursdays in the Russell Hotel in London), where "admissions tutors would need to receive applications at a similar stage".
Simon Berry, careers adviser at Harrow, said he and his colleagues had been very surprised by the circular: "It's extraordinary that the school is being told to do this. I assumed that the universities themselves would tell us when things should be sent off. We have always said that applications for gold-dust courses like medicine should go early, but for everything else we aim for half term."
At Oundle, the headmaster, David McMurray, said: "We always try to get ours out by half term, around October 20, but now we're hurrying it along for those applying for shortage subjects or the Russell Group of universities. Universities are making offers before the deadline - they don't sift through the whole pile right at the end."
At Hinchingbrooke school, a comprehensive in Cambridgeshire which sent 52 per cent of the sixth form on to higher education in the 1994 round of applications, deputy head Gordon Boyd also said that his school used the half-term deadline.
The HMCGSA survey also revealed that pupils who named Russell Group universities -which include Birmingham, Bristol, Durham, Edinburgh, Warwick and some London University colleges - in all six of their choices would frequently only get offers from two. Some Oxbridge candidates would not be considered by the others at all.
John Dunford, president of the Secondary Heads Association, who is also on the board of the Universities and Colleges Admissions Service, said that it was important that the university entrance process was as open as possible. "It could be more open if universities were not able to see the other universities students had applied for. The survey shows that an Oxford or Cambridge application will skew others' views."
He was not particularly surprised by the advice about early applications. "One can imagine the problem for an admissions tutor faced with hundreds of applications for 20 places. We've known for a long time that courses start giving out offers long before the final date." SHA, he said, is arguing for a post-A-level applications process.
Tony Higgins, chief executive of UCAS, agreed that a post-examination admissions system might be the answer. "It's a burning issue at the moment. I regret that young people are being forced to apply earlier and earlier, but it's tactically wise."
The issue of post-examinations applications will be debated at the annual conference of the Committee of Vice Chancellors and Principals in Belfast next week.