Taiwan said Monday that three retired military officers have been arrested on suspicion of leaking military secrets to China, in what legislators describd as one of the island's worst espionage cases.

Chang Chih-hsin, formerly in charge of political warfare at the navy's METOC (meteorology and oceanography) office, is among those held, the defence ministry said in a statement.

"Chang, who initiated contacts with Chinese mainland officials while still serving in the navy, was suspected of luring his former colleagues and making illegal gains," it said.

Defence ministry spokesman David Lo confirmed two other former military officers have also been arrested in the case.

Lo did not say what kind of military information Chang allegedly sold to China but played down the damage to Taiwan's security, saying he had limited access to sensitive information.

Apple Daily newspaper said a total of eight former military officers had been arrested.

It quoted a retired naval general as saying the naval METOC kept highly classified information such as maps and charts used by the island's submarines and other warships.

If China had such information, it could learn more about the operations of Taiwan's submarines, the ex-general warned.

"This has gravely endangered Taiwan's security. It's a shame for the military," legislator Lin Yu-fang of the ruling Kuomintang party told reporters.

Wung Ming-hsien, a professor at Taipei's Tamkang University, said increased contacts between Taiwan and its former bitter rival China over the past few years had blurred the line between friend and foe.

"The case again indicates that because of closer civil contacts across the Taiwan Strait, the national identity is collapsing, therefore posing a great threat to Taiwan's security," he said.

Legislator Tsai Huang-lang of the main opposition Democratic Progressive Party said the case shows China has "fully infiltrated" the armed forces.

"Taiwan's submarines could be easily destroyed and became iron coffins should war break out across the strait," he said.

Relations have improved markedly since President Ma Ying-jeou of the China-friendly Kuomintang came to power in 2008 on a platform of strengthening trade and tourism links.

But China still regards Taiwan as part of its territory awaiting reunification, by force if necessary.

In July last year a Taiwanese general lured by a honey trap into spying for China was jailed for life in one of the island's worst espionage cases for half a century.