After the debut of the diesel Bentayga, packing a triple-charged 4.0-liter V-8 engine generating 429 hp and 664 pounds-feet of torque, there was really only one unexplored powertrain option for Bentley's luxury tank: a hybrid.

A plug-in hybrid Bentayga is indeed in the works, and prototypes have been spotted testing in the U.K., Auto Express reports. Yes, ultimately this means the Bentayga will be able to take advantage of diamond lanes reserved for electrics and hybrids, traveling along with Toyota Priuses and Nissan Leafs.

Don't write off this version just yet, as it sounds like it'll be the perfect antidote to pangs of luxobarge guilt: The prototype Bentayga has been testing with the Porsche Cayenne S E-Hybrid's powertrain, which pairs a 3.0-liter twin-turbo V-6 churning out 333 hp with an electric motor producing 95 hp, for a total system output of 416 hp in the current Cayenne.

We'd expect a production version to use the newer 2.9-liter V-6/motor combo from the new Panamera E-Hybrid, generating a combined 462 hp and 516 pounds-feet of torque with the help of a 100-kilowatt electric unit.