WASHINGTON—The U.S. Steelworkers union plans to file a petition Thursday demanding a government investigation into China's policies to support its fast-growing green-technology sector.

"China has utilized hundreds of billions of dollars of subsidies, performance requirements, preferential practices and other trade-illegal activities to advance their domination of the sector," the union said in a statement announcing the case.

In its latest broadside against China's trade policies, the 1.2-million-member labor group said it would file a "section 301" case with the U.S. Trade Representative's office. The administration would have 45 days to decide whether to investigate the allegations of unfair trade practices, and a year to complete the probe and determine a course of action.

A USTR spokeswoman said the office would review the petition. "USTR has been discussing issues surrounding China and green technology with the steelworkers and with other stakeholders, and will continue to do so," she said.

The filing, totaling more than 5,800 pages, will allege five major areas of Chinese protectionist practices that the Steelworkers claim threatens the U.S. renewable and alternative-energy sector.

The Steelworkers have been among the most active critics of China trade policy, with some success. President Barack Obama decided in the union's favor last September in a landmark case to impose a special safeguard provision to impose tariffs on Chinese tire imports. The group has also brought several successful countervailing and antidumping cases against Chinese imports.

Amid increasing calls in Congress to take action against China's tightly managed currency policy, the Obama administration sent a high-level delegation to Beijing over the weekend to ease tensions on trade and other issues.
SEPTEMBER 9, 2010, 10:46 A.M. ET