TAIPEI - A close relative of Taiwan leader Chen Shui-bian resigned from the ruling party Thursday, amid growing concern that his alleged involvement in a stock scandal could further undermine Chen's authority.

The decision by Chen's son-in-law, Chao Chien-min, followed calls from Democratic Progressive Party lawmakers to expel him from its ranks.

Chao has acknowledged that his mother, Chien Shui-mien, bought stocks in the ailing Taiwan Development Corp. just before their value skyrocketed, but has denied accusations of insider trading. Prosecutors interviewed company managers over the allegations Wednesday.

On Thursday, Chao issued a statement announcing his resignation from the DPP "to avoid causing any more harm to the party."

There was no immediate reaction from DPP lawmakers to Chao's decision.

Chao's alleged involvement in the stock scandal comes as Chen sags badly in opinion polls.

An poll published by the mass-circulation United Daily News Thursday gave him an approval rating of 20 percent, the lowest in the newspaper's surveys since his re-election in March 2004.

The paper said it interviewed 823 adults by phone Wednesday for the survey, which had a margin of error of 3.4 percentage points.

The Chao scandal is not the only one buffeting Chen's inner circle.

Opposition lawmakers say that Chen's wife, Wu Shu-chen, profited from insider trading and received vouchers from a department store after intervening in its sale. Chen says his wife is innocent of the charges.

Last month, a former aide of Chen Shui-bian was arrested for his alleged involvement in another high-level corruption scandal, adding to the DPP's growing worries over damage to its public image.