The council's education committee agreed last week to link the project to the new primary school at Newton Mearns to create a single PFI package worth Pounds 8 million.
Councillors also agreed to seek increased borrowing consent in the normal way, although the Scottish Office is unlikely to increase East Renfrewshire's capital programme which stands at only Pounds 1 million for all council services except housing.
If the council is forced down the PFI route, it could delay both projects by a year, Jim Fletcher, the committee's convener said.
The move to add 24 classrooms to St Ninian's confirms East Renfrewshire's "big is beautiful" approach. It would take the school roll from 1,320, which is 30 per cent above capacity, to its projected figure of 1,636 by 2005. The primary at Newton Mearns would hold 924 pupils and be the largest in Scotland.
Planning officials have warned of public objections based on loss of open space and increased volumes of traffic. But the education committee has decided that tinkering with admissions to the school to ease pressure on accommodation must come to an end.
A limit of 231 has been imposed on the intake of first-year pupils, posing an immediate crisis next session when 243 pupils are expected from the school's five associated primaries (two of which are in Glasgow).
Four pupils who live in the St Ninian's catchment area but do not attend a local primary have been turned away this year. If no action is taken the position would deteriorate to such an extent by 2005 that 278 pupils entitled to attend would have been refused admission.