Toyota has already confirmed it will release a plug-in hybrid vehicle powered by next-generation lithium-ion batteries in Japan, the U.S. and Europe sometime next year but production of these will be limited and only commercial and government fleets are expected to receive the cars. Mass production of Toyota’s plug-in hybrid isn’t expected to kick in until 2012, according to latest reports.

Japan’s Nikkei business daily is reporting that Toyota will build roughly 20,000 to 30,000 plug-in hybrids in 2012, almost two years after closest rival General Motors starts producing its Chevrolet Volt.

A version of Toyota’s plug-in hybrid based on the conventional Prius hybrid will go on sale in Japan this month to fleet customers, and according to the same reports it will be priced at ¥4.59 million (approximately $47,800). It will run on lithium-ion batteries supplied by Panasonic EV Energy and will reportedly be able to travel up to 18.6 miles on electric power alone.

Toyota wasn’t willing to confirm any of the latest detail but the automaker has previously confirmed that trials of 500 Prius plug-in hybrids will start at the end of the year. The cars will be leased to government and commercial fleets, with 200 set to stay in Japan, a further 150 headed to Europe and the final 150 reserved for the U.S. market. The Prius plug-in hybrids are already averaging 65mpg in the combined cycle during initial testing - that's a 30% improvement over the standard 2010 Prius, recently certified as the most efficient hybrid in America.