Microsoft has hired Cap Gemini Ernst & Young to provide free consultancy for an East London council.
Newham Borough Council is reviewing its open source options and running trials on desktop and server-based systems.
In response, Microsoft offered free consulting, which is aimed at proving it has the best products.
A decision on procurement is likely by Christmas, according to the council's IT director Richard Steel. Its decision could have an impact on the uptake of open source in the public sector.
Last week Newham was named by the Office of Government Commerce and the e-envoy as a site for one of nine open source "proof of concept" trials.
The public sector is becoming a battleground for open source, with Peter Gershon, chief executive of the OGC, promising to create a level playing field between open source and proprietary software in government procurement.
Newham's decision to explore open source was driven by last year's controversial changes in Microsoft licensing arrangements and the fact that software developments are forcing hardware upgrades, said Steel.
Matt Lambert, Microsoft's director of government affairs, Europe, said it was funding Cap Gemini to prove the value of its own products to the council. "Newham is seen as a technology leader. It is an important account for us and we are trying to prove that we have the best offering."
Microsoft was confident that it was competitive if organisations compare total cost of ownership of its products with open source rivals, said Lambert. He declined to reveal how much it was paying Cap Gemini Ernst & Young for the consultancy.
Microsoft's tactics in Newham were "normal business", he said and it would respond to open source initiatives at other councils on a case-by-case basis.
Mike Thompson, principal research analyst at Butler Group, questioned whether it was normal to give free consulting in such circumstances. "Suppliers usually compete on cost and services, not on consulting," he said. "Microsoft will have considered the knock-on effect on the market."
The next IT Trends survey from local authority IT directors' organisation Socitm is likely to show about 20% of councils are selectively using open source, 50% have it under active consideration, and 25% have rejected it.
The OGC welcomed Microsoft's intervention. "The more research, the better informed the choices the public sector can make," said a spokesman.