MANILA - Former Ombudsman Conchita Carpio Morales warned of "treason from within" in the West Philippine Sea dispute with China.

"A nation can survive its fools, and even the ambitious. But it cannot survive treason from within," Morales said, quoting Roman statesman Cicero, at a forum in Taguig City on Saturday.

"An enemy at the gates is less formidable, for he is known and carries his banner openly, but the traitor moves amongst those within the gate freely, his sly whispers rustling through all the alleys, heard in the very halls of government itself," she added.

Morales also quoted former President Manuel L. Quezon, who once said: "Let not our children be mere tenants and trespassers in their own country."

Morales, likewise, referred to the Philippines' international arbitration victory in its maritime dispute over China in 2016, expressing her hope the country asserts its gains in The Hague.

"The writings are on the wall. The ensuing period of years is the litmus test of how this nation is going to assert the gains it has painstakingly fought in The Hague," she said. "I earnestly hope this nation will not fail its people."

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Morales, who retired last July, delivered her speech days after the Philippines signed a memorandum of understanding on joint oil and gas development with China.

The deal, signed during Chinese President Xi Jinping's state visit to Manila, is essentially an "agreement to agree," as described by Foreign Affairs Secretary Teodoro Locsin Jr.

• READ: Philippines-China oil and gas development deal

Acting Chief Justice Antonio Carpio, who was also present at the forum, said China's historical claim over the West Philippine Sea can be considered "false news of the century."

During the forum, Carpio showed old maps which he and the Philippine government presented before the arbitration court in The Hague.

"This is the narrative China uses to claim the South China Sea... This is the false news of the century, a gigantic fraud in the human race," Carpio said.

Carpio was one of the lawyers who represented Manila in its arbitration case against Beijing for maritime rights in the regional waters.

The Philippines, China, plus 4 other claimants, have for decades been embroiled in a dispute over the South China Sea. It reached a critical point during the presidency of Duterte’s predecessor, Benigno Aquino III, who approved the filing of a case against Beijing before the Permanent Court of Arbitration in 2013.

The tribunal essentially ruled in favor of Manila in July 2016, declaring China’s expansive nine-dash line claim to the sea invalid. Beijing has ignored the landmark ruling.

• Philippines beats China in international court