The Association of Commonwealth Universities could be seriously weakened if any more British universities drop out to save money, its new secretary general has warned.
At least three have withdrawn in the past year - although one has since rejoined.
Scotland's oldest university, St Andrews, is the most dramatic example, especially as it has not been severely hit by funding cuts.
A spokesman said: "The university annually reviews external associations to ensure it achieves value for money. It decided not to be part of the ACU, not only this year but last."
Staffordshire University blamed the tight financial situation. "We have had to look at our membership of organisations. The ACU was one which, when we looked at it on a cost/benefit basis, was not a priority."
Last year Lancaster University briefly dropped out, also for financial reasons, but has since rejoined. Its annual subscription is approximately Pounds 1,500.
Writing in the ACU's annual report, Michael Gibbons, the secretary general, described the UK withdrawals as "disturbing" and unprecedented.
"That they are occurring now no doubt reflects the seriousness of financial situations confronting those particular institutions. Nonetheless, heretofore it has always been possible to claim that every UK university was a member of the ACU. If the trend were to continue, it could seriously weaken the association."
A small number of the former polytechnics remain outside the organisation, but at least one, the University of Central England at Birmingham, is now joining. Overall ACU membership has fallen slightly to about 450, causing a slight drop in subscription income from Pounds 883,000 to Pounds 870,000.
Opinion, page 13