The four defendants were among five people expelled from Lai's ruling Democratic Progressive Party (DPP) -- which advocates for Taiwan's sovereignty -- last month for suspected espionage.

The fifth person is still under investigation, a prosecutor told AFP.

China claims democratic self-ruled Taiwan is part of its territory and has threatened to use force to annex it. Taipei accuses Beijing of using espionage and infiltration to weaken its defences.

The four people have been charged with violating the Classified National Security Information Protection Act and National Security Act after they allegedly shared state secrets with China, the Taipei District Prosecutors Office said in a statement.

Other alleged offences include money laundering.

The espionage happened over a "prolonged time", included the sharing of "important and sensitive diplomatic information" and caused "significant harm to national security", prosecutors said.

Prosecutors are seeking prison sentences ranging from five to more than 18 years, and the confiscation of more than NT$8.3 million (US$277,000) in illicit gains.

The heftiest sentence is for a defendant surnamed Huang, who allegedly transmitted information to China "via a specific messaging app".

Huang previously worked for a New Taipei City councillor, the semi-official Central News Agency (CNA) said.

Among the other defendants is a person surnamed Ho, who CNA said is a former aide to National Security Council chief Joseph Wu when he was foreign minister.

Huang and Ho have denied the allegations against them, prosecutors said.

The other two defendants, including the person who had worked for Lai when he was vice president and then president, have confessed.

Beijing and Taipei have been spying on each other for decades, but experts say the threat to Taiwan is greater given the risk of a Chinese invasion.

Taiwan's National Security Bureau said previously 64 people were prosecuted for Chinese espionage last year, compared with 48 in 2023 and 10 in 2022.

In 2024, they included 15 veterans and 28 active service members, with prison sentences reaching as high as 20 years.