The four people were charged in June, a month after they were expelled from Lai's ruling Democratic Progressive Party (DPP) -- which advocates for Taiwan's sovereignty - over suspected espionage.

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 Taipei District Court said in a statement the four were convicted of violating the Classified National Security Information Protection Act for leaking state secrets to China.

"The information they spied on, collected, leaked and delivered involved important diplomatic intelligence...which made our country's difficult diplomatic situation even worse," the court said.

The espionage happened "over a very long period of time," including sharing itineraries of high-level officials such as the foreign minister, which "endangers the country's diplomatic security and is highly condemnable".

The heftiest ten-year sentence is for Huang Chu-jung, who previously worked for a New Taipei City councillor.

According to the statement, Huang mixed public information with "secrets and confidential information" he received from Ho Jen-chieh, an aide to then foreign minister Joseph Wu, to write analysis reports and "sent to Chinese agents using encrypted software".

Ho was sentenced to eight years and two months in prison.

Huang and another defendant Chiu Shih-yuan, who received a jail sentence of six years and two months, were also convicted of laundering around NT$7.2 million (US$236,600) in illicit gains.

The fourth defendant Wu Shang-yu, who had worked for Lai when he was vice president and then president, received a four-year prison term.

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, with prison sentences reaching as high as 20 years.