U.S. traffic safety authorities have launched an investigation into Toyota Motor Corp's <7203.T> 2000 and 2001 model year Tundra pickups following complaints alleging that severe frame corrosion caused brake system failures or spare tire separation.

The probe follows the largest U.S. recall for Toyota announced just a week ago and comes at a critical time for the company as it scrambles to hold on to a hard-earned reputation for safety and quality that helped it surpass General Motors Co as the world's top automaker last year.

The National Highway Traffic Safety Administration said on Tuesday it had received 20 complaints related to the Tundra, 15 alleging that the underbody-mounted spare tire separated from the rear cross member and five alleging damaged brake lines due to corrosion on the driver's side rear cross member.

An estimated 218,000 units are involved, the NHTSA said.

A Toyota representative was not immediately available for comment.

Toyota said last week it would recall some 3.8 million vehicles in the United States because of the risk that a loose floormat could force down the accelerator, a problem suspected of causing crashes that have killed five people.

Analysts said the floor mat recall likely would involve a minimal financial impact, but could have an impact on Toyota's reputation in the U.S. market.

Meanwhile, A U.S. trade panel that hears patent disputes said on Tuesday it would investigate a claim that Toyota infringed on patents in producing its popular hybrid vehicles.

(Reporting by Soyoung Kim, editing by Dave Zimmerman)