TAIPEI - Taiwanese prosecutors on Monday indicted the son-in-law of "president" Chen Shui-bian on insider trading charges, further eroding Chen's leadership position.

Prosecutors indicted Chao Chien-min, the alleged centerpiece in an elaborate insider trading scheme involving a Taipei property company. They say they will demand an eight-year jail sentence.

The case against Chao is among a series of high-profile corruption charges involving Chen's family and inner circle. Chen himself has not been implicated, but Taiwan's opposition has demanded that he step down, saying that the corruption allegations have tarnished his credibility.

Chao, a 34-year-old physician, was arrested in May on suspicion he used insider information to profit on the purchases of shares in partly state-owned property company Taiwan Development Corp.

That scandal followed opposition charges that Chen's wife, Wu Shu-chen, received free vouchers from the management of an upscale Taipei department store, and may have played a role in its takeover by a businessman whose bid was less than that of at least one rival suitor.

Chen's office has denied those charges.

Last week prosecutors indicted former senior Chen adviser Chen Che-nan on insider trading charges and allegations that he took bribes from a businessman to help resolve his legal problems.

The two Chens are not related.

Amid the cascade of scandals, Chen Shui-bian's position has gone into free-fall. His public approval ratings now hover around the 20 percent mark.

Last month the opposition Nationalist Party failed in a bid to muster the required two-thirds votes in the island's Legislature to hold a referendum of Taiwanese voters on whether Chen should be removed from office.

However, opposition leaders have kept up their efforts to force Chen from office, organizing island-wide petitions calling for his resignation, and maintaining a drumbeat of criticism in anti-government newspapers.

Chen was re-elected in 2004 and is due to complete his second and final four-year term in 2008.