Production of Toyota's fourth-generation Prius has been delayed until December 2015 as engineers work to perfect the car's new vehicle architecture and hybrid system, industry sources told Automotive News.

The new Prius, which was earlier slated to begin production in the spring, has been code-named 690A. A confirmation vehicle version, which is a prototype built about a year before to be used for testing performance, should be put together in November.

Toyota hasn't explained the reasons behind the around six-month wait until production; however, someone familiar with the matter said the car was being adjusted for maximum fuel economy, according to Automotive News. Some of the tweaks may include changes to the body and chassis.

The delay may be partly due to the new technologies that will be tested in the Prius, including a modularized vehicle architecture and new hybrid system.

Satoshi Ogiso, managing officer for product planning, said the next Prius will serve as a test bed for new technology, Automotive News reported.

Toyota is aiming for fuel economy that is at least 10 percent improved from the third-generation Prius.

"We need to prioritize fuel efficiency," Ogiso said.

Toyota engineers are working to reboot the next-generation hybrid system to support a wider variety of engines and vehicle types. One way fuel efficiency will improve is through a new gasoline engine with thermal efficiency rates above 40 percent, topping Toyota's most efficient engine so far, the current Prius engine at 38.5 percent. Thermal efficiency measures how much energy is captured to power the car versus how much is lost through escaping heat. 

First launching in 1997, the Toyota Prius is the top-selling hybrid worldwide. The Japanese automaker sold more than a million hybrid vehicles in 2013, reaching that sales figure for the second year in a row.

The plug-in version of the Prius should begin production in October 2016.  

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