The Society of IT Management (Socitm) issued the call ahead of a Commons Public Accounts Committee report on public sector purchasing of software licences due out later this week.
The call follows the agreement struck by Newham Council last month that could save it £1m, after trialling Linux. Some local authority IT directors believe Microsoft can be persuaded to offer concessions to other councils that improve on terms agreed between the supplier and the OGC.
Socitm pioneered a licensing agreement with Microsoft that cut costs for local authorities, and collaborated with the OGC to win deals from Oracle, Sun and IBM for the whole public sector.
But Bob Griffith, international secretary at Socitm, who was closely involved with the Microsoft deal, said although the deals had saved taxpayers money, local government remained a "junior partner" to the OGC. "The OGC is now playing the dominant role in software licensing negotiations and there obviously needs to be more of a sharing role," he said.
Giving evidence to the committee, Peter Gershon, chief executive of the OGC, said he needed the combined purchasing power of the whole public sector to get the best deals. Griffith said open source options could be used as a bargaining tool by public sector negotiators discussing licensing terms with Microsoft.