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The sale is planned to be launched during the three months to December, the sources said, with one of the sources saying that investor roadshows could start in mid-September.
The government, which has a 68.94% stake in ONGC, is considering selling a 5% stake, worth about $3 billion at the current market price.
If completed, the sale could raise more than a quarter of the $10.5 billion target for asset sales this financial year announced by Finance Minister Arun Jaitley to help to bridge the budget deficit.