"President Michel Suleiman warns against any unilateral decisions Israel may take on maritime borders which would be a breach of international law, as is Israel's habit," Suleiman's office said in a statement.
Suleiman also said the issue would be up for talks at the first meeting of Lebanon's new government, which last Thursday won a vote of confidence in parliament.
While Prime Minister Najib Mikati's government is dominated by Shiite militant group Hezbollah, the last Western-backed Lebanese government had taken a similar stand in the growing conflict over offshore gas reserves.
Israel's cabinet on Sunday approved a map of the country's proposed maritime borders with Lebanon to be submitted for a UN opinion.
The proposed map lays out maritime borders that conflict significantly with those suggested by Lebanon in its own submission to the United Nations.
Lebanon's Energy Minister Gebran Bassil said Beirut will not give up its maritime rights and accused Israel of "violations of (Lebanese) waters, territory and airspace, and today our oil rights."
Israel for months has been moving to develop several large offshore natural gas fields in the eastern Mediterranean, some shared with Cyprus, that it hopes could help it to become an energy exporter.
But its development plans have stirred controversy with Lebanon, which argues the gas fields lie inside its territorial waters. Israel does not have officially demarcated maritime borders with Lebanon, and the two nations remain technically at war.