Confirming a poorly kept secret, Nissan Motor Co. Ltd.’s Infiniti brand says it will retail a hybrid-electric version of the M sedan in the U.S. in spring 2011.

The M35 HEV will trail by a year the debut of the non-hybrid V-6 and V-8 M models. They are due to go on sale next spring in the U.S.

The model features Nissan’s own hybrid system, which has one electric motor mated to a twin clutch. The first clutch is placed between the 3.5L V-6 and the electric motor. Infiniti says a “consistent driving feel under all conditions with linear performance” is a benefit of this configuration.

The electric motor can propel the V-6 in “power assist” mode when a driver needs maximum acceleration, and also acts as a generator.

Infiniti says the M can run solely on electric power but is not specific as to the conditions under which this is possible.

A laminated lithium-ion battery pack will store energy and is the same size as conventional batteries but with twice the power, Infiniti says.

The M hybrid will be retailed in all 33 countries Infiniti sells vehicles.

Nissan and Infiniti officials toyed with the notion of a diesel-engine model for the U.S. instead of a hybrid. However, Infiniti says only Western Europe will receive the upcoming high-performance diesel M.