Forcing the two major European economies to meet the EU's tough deficit criteria "would likely depress activity further and even risk tipping the euro area into another recession," it said.
France and Italy's "slower pace of structural fiscal adjustment... Proposed in their 2015 budget plans seems appropriate," said the Organisation for Economic Cooperation and Development, which provides economic analysis and advice to its 34 industrialised members.
Its assessment came as an EU source said the Commission would give the two countries until the spring to implement tough reforms, delaying a final verdict on national overspending originally set for this week.
The decision, to be approved by the Commission later today, risks angering an impatient Germany, frustrated with the slow pace of reform in Paris and Rome.
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Last month, France and Italy barely avoided having their budgets humiliatingly sent back for serious breaches.
According to the plans, the Commission will divide eurozone countries into groups, with France and Italy, but also Spain and Portugal, identified as non-compliant of the EU's budgetary rules.
The OECD lowered its forecast for global growth this year by a tenth of a percentage point to 3.3 per cent. For 2015 it cut the forecast by two tenths of a point to 3.7 per cent growth.
The OECD left in place its forecast for the 18-nation eurozone to expand by 0.8 per cent this year and by 1.1 per cent in 2015.
"Growth is set to be stronger in the United States and the United Kingdom than in the euro area and Japan," the report said, adding that unemployment "will remain particularly high in the euro area".
"This macroeconomic prognosis leaves us with a keen need for both continued supportive macroeconomic policy, as well as tailored structural reforms to raise both demand and supply throughout the global economy," it said.