One of Taiwan's top negotiators on China policy is being investigated on suspicion of leaking secrets after resigning, his superior said Wednesday, in the latest political scandal to rock the island.

Wang Yu-chi, minister in charge of Taiwan's Mainland Affairs Council, said his former deputy Chang Hsien-yao is being investigated following a tip-off he received in late July accusing the latter of leaking confidential work-related information.

"I can't disclose the content of the tip-off ... We have provided relevant information to the Investigation Bureau so they can utilise that in their further investigation," Wang told reporters.

The Investigation Bureau is tasked with countering Chinese espionage against Taiwan as well as criminal activity.

The mainland affairs council said Tuesday Chang has been under investigation since stepping down over suspicions related to "national security". It did not elaborate.

Wang's comments came after Chang went on local television late Tuesday to defend himself, claiming that he was persecuted over "concocted" allegations and was "sold out" by his superiors.

Local media suggested that Chang might have leaked classified information during previous talks with China, including the island's bottom line in negotiations.

The scandal came after China's top official in charge of Taiwan affairs, Zhang Zhijun --the most senior Chinese official ever to visit the island -- paid a landmark four-day visit in June.

Ties between Taiwan and China have improved markedly since 2008 when President Ma Ying-jeou, of the China-friendly Kuomintang party, came to power.

Observers said Chang's removal could complicate Ma's pro-Beijing push, which has already been hampered by major student-led protests in Taipei against a trade pact earlier this year.