Rumors of an MPV or minivan based on Toyota's popular Prius have been swirling around for quite some time. Given that current 5-seat Prius is capable of 50 mpg, it's seems reasonable to think that a slightly larger vehicle with additional seating/cargo capacity would be welcome in the marketplace (with a slight hit to overall fuel economy).

Today, Edmunds Inside Line has some of the first spy photos of the MPV counterpart to the Prius hatchback. According to the publication, the vehicle will seat seven passengers and will be similar in size to such vehicles as the Mazda 5 and the Kia Rondo. The Mazda 5 in particular is quite popular with small families as it is closer in size to the original Chrysler minivans that debuted in the mid-80s instead of the gargantuan Siennas, Odysseys, and Caravans prowling the streets today.

Given that the MPV will share much in common with the standard Prius, we expect to see the same 1.8-liter gasoline engine used and a new lithium-ion battery pack for added power and range (while at the same time saving weight). It shouldn't be too difficult for the MPV to achieve greater than 40 mpg combined (city/highway), but we'll just have to wait for the final EPA numbers to come in when the vehicle is released next year.

When the Prius MPV does hit U.S. streets, it will likely be joined by a similar offering from General Motors: the Chevrolet Volt MPV5. The "crossover" variant of the standard Volt sedan seats five people and can travel up to 35 miles on battery power alone.

Updated 10/11/2010

Toyota has just posted a teaser shot of it upcoming Prius MPV on its Facebook page.