Swiss prosecutors 'have requested assistance in their investigation and we have immediately started our own probe', he told reporters without elaborating.

Taiwan's justice ministry said the request, received on July 31, was from the Swiss side which was investigating suspected money laundering by Chen's son Chen Chih-chung and daughter-in-law Huang Jui-ching.

Copies of the Swiss documents obtained by Kuomintang lawmaker Hung Hsiu-chu showed that Chen Chih-chung and his wife had transferred some US$31 million (S$43.7 million) to Huang's Swiss bank accounts in 2007.

The ex-leader's office said on Friday US$20 million from his campaign funds were wired overseas but rejected the money laundering allegations.

Chen himself apologised on Thursday for failing to 'fully declare' his past campaign funds, saying his wife Wu Shu-chen had transferred some money abroad without his knowledge.

Chen was elected mayor of Taipei city in 1994 but lost his reelection bid in 1998. Four years later he won presidential elections and in 2004 he was narrowly re-elected for his second and final term.

He claimed that the funds his wife had sent overseas had no relation with a probe into whether he was guilty of pocketing presidential special expenses.

The local Next magazine also reported this week that Chen had allegedly transferred some NT$300 million of ill-gotten funds to overseas bank accounts of his daughter-in-law and her family since 2006.

The magazine claimed that the authorities discovered the suspicious money transactions while investigating a high-profile embezzlement case implicating the former president and his wife.

The ex-president was questioned on Tuesday for allegedly embezzling NT$14.8 million in special expenses from the government during his tenure.

He was named a suspect in the case in 2006 but escaped immediate prosecution because of presidential immunity.

His wife is on trial for corruption and document forgery in the same case.

Chen has admitted using false receipts to claim money from the state, but insisted those funds were used for 'secret diplomatic missions' and not his personal benefit.

Nevertheless, prosecutors found that at least NT$1.5 million had been spent on diamond rings and other luxury items for his wife. -- AFP