A senior military intelligence officer has been arrested in Taiwan for being a double agent for China, Taiwan's defence ministry has said.
The agent allegedly passed significant amounts of intelligence to China, potentially endangering the safety of other spies.
The defence ministry said the agent was a senior officer who had been under investigation for the past four years.
He was arrested on Monday on suspicion of selling state intelligence to China.
The defence ministry declined to give further information, but said it had taken measures to limit the damage caused by the officer.
Taiwanese media has reported that he had given China a list of other spies working on the mainland.
One newspaper quoted an unnamed military source as saying the leaks had compromised important Taiwanese intelligence networks in China and that its agents there were now running for their lives.
The officer was working with a Taiwanese businessman gathering intelligence for Taiwan, according to the reports.
That businessman was later arrested, allegedly tortured, and persuaded by China to not only serve as a double agent, but to convince the senior officer to also become a double agent, the reports said.
The senior officer had allegedly received US$100,000 (£62,000) since 2007, when he began forwarding classified information to the businessman, to pass on to China.
The businessman had also been arrested in Taiwan, reports said.
The case surfaced as relations between former rivals Taiwan and China reached their best in 60 years.
But despite closer economic ties in recent years, distrust between the two remains and China still considers Taiwan its territory.
Dozens of Taiwanese spies have been jailed in China in recent years. Many of them were businessmen recruited to gather intelligence for Taiwan.