The spokesman quoted Rajapaksa as saying the firm had also begun drilling the second test well.
The discovery announced Sunday is the first since intermittent exploration for oil and gas began in the 1970s. He made the announcement in a speech at a meeting for local government polls in Kandy in the central hills.
The spokesman quoted Rajapaksa as saying an oil exploration firm drilling test wells in offshore waters had informed the government about the discovery Sunday morning.
"I was getting ready to come to Kandy early this morning when I got good news that I thought of sharing with the people in Kandy," Rajapaksa was quoted as saying.
"The company doing test drilling in the Gulf of Mannar informed me this morning that they have found a huge deposit of (natural) gas in the sea bed.
"This will help us to meet the country's energy requirements.
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Rajapaksa did not give further details but Cairn India began drilling its first test well in the Mannar Basin off the north-west