Toyota has decided to delay the start of production for the fourth-generation Prius from next spring until December 2015, as engineers work out 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 version of its hallmark hybrid, code-named 690A.

That has been pushed back to December.

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 unclear.

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.