The trustee administering $541 million in bonds from Suntech Power Holdings Co sent the solar company a notice of default on Saturday as the deadline to redeem the notes passed, a person familiar with the matter said.

The move allows bondholders to sue the Chinese company in US courts, according to the person, who asked not to be named because the information hasn't become public. A default would be the first by a Chinese mainland company. Suntech said holders of 63 percent of the bonds agreed to delay exercising their rights for two months until May 15.

"We believe it is in the best interests of all of our bondholders to come to a consensual restructuring and encourage bondholders that are not signatory to the agreement to join this process," Suntech said in a statement released by its public relations adviser, Weber Shandwick in Shanghai.

The default notice brings Suntech a step closer to court intervention in its efforts to renegotiate payments on the bonds, which were issued in 2007 and matured on March 15. Suntech is "likely" to enter bankruptcy, Aaron Chew, an analyst at Maxim Group LLP in New York, said in a March 14 note.

Bankruptcy may wipe out Suntech's shareholders, including founder Shi Zhengrong, who was ousted as chairman last week and still controls about 30 percent of the company.

Suntech, based in Wuxi outside Shanghai, said on March 14 that it had no plans to make the final payment in time for the deadline. It hasn't reported a profit since the first quarter of 2011 after a slump in the cost of solar cells gutted margins.

Since July, Suntech has hired UBS AG to restructure its debt and downgraded its outlook for sales. While the company announced on March 11 that a majority of its bondholders are willing to wait for repayment, some that weren't involved in the so-called forbearance agreement are threatening to sue.

"A group of bondholders is presently organizing and is preparing to take action," James Millar, a partner working on bankruptcy at the law firm WilmerHale in New York, said on March 15. He expects to represent several Suntech bondholders. "Each bondholder has a right to bring an action to collect all principal and interest that remains unpaid."

Support from 25 percent of the bondholders may be needed to take action against Suntech, according to a prospectus for the notes filed with the Securities and Exchange Commission in June 2008. Wilmington Trust is acting as trustee for the bonds and has the authority to declare whether the bonds are in default. Officials at the trust had no immediate comment.

Millar said holders don't need to meet the 25 percent threshold before pursuing their rights to be repaid.

"Holders of a majority in aggregate principal amount of the outstanding notes will have the right to direct the time, method and place of conducting any proceeding for any remedy available," according to the prospectus. It also says "no holder will have any right to institute any proceeding" unless at least 25 percent of the principle holders make a written request to the trustee of the bonds.

The document also says 60 days must elapse after the written notice before holders have rights to "institute any proceeding".