TOKYO -- Toyota Motor Corp. has delayed the start of production for the fourth-generation Prius from next spring until December 2015, as engineers sweat the details of a new vehicle architecture and the company's next-generation hybrid system.

Toyota initially targeted a spring manufacturing launch for the next iteration of its hallmark hybrid, code-named 690A. That has been pushed back to December, people familiar with the matter say.

The Prius confirmation vehicle, a prototype typically built 12 months before Job 1 to ensure performance, is scheduled for around November 2014.

Production of the plug-in version is expected to begin in October 2016.

Reasons for the roughly half-year delay are murky. One source said engineers were tweaking the car to ensure maximum fuel economy, possibly adjusting body and chassis issues.

A Toyota spokesman said the company could not comment on future product plans.

Hybrid czar Satoshi Ogiso, managing officer for product planning, also declined to comment on timing. But he told Automotive News the car will be a test bed of critical new technologies, including Toyota's modularized vehicle architecture and next-generation hybrid system.

Nailing the next Prius is paramount, so engineers may be forgiven for taking time to get it right. The car, which premiered in 1997, is the world's best-selling hybrid and a technology showcase.