Lamborghini and Bentley are both planning to leap into the unknown by building big, soft-road four-wheel drives by 2015 from a pair of new SUV architectures being developed by the Volkswagen Group. Sources have revealed the top-secret platform development will see a luxurious and a sporting chassis being created for use around the group, with the luxury architecture going to Volkswagen, Audi and Bentley, while Porsche and Lamborghini would share the sporting one. The architectures are believed to be related, but only to a point, with the sportier platform geared up for extra stiffness and torsional rigidity and lighter weight. Crunch time is approaching as the viability of both super-expensive off-roaders is due to be determined by the end of the year. Both Lamborghini boss Stefan Winkelmann and Bentley CEO Wolfgang Durheimer have spoken openly about their desire to flesh out their line-ups with third full-model lines, though only Lamborghini has any off-road heritage. While Durheimer admitted Bentley had no off-road DNA, he insisted a business plan for the SUV, already jokingly dubbed the \"TransContinental GT\" by some insiders, would be finalised by the end of the year. \"The decision will be made when the business plan is finished and the business plan is one of the most important things we\'re working on,\" Durheimer said. \"If you are a Bentley driver, you own five cars, and 100 per cent of Bentley buyers own an SUV, basically,\" he said. \"If they like the craftsmanship, we have a good chance to replicate our success with an SUV. There is no offer in the segment that we had to fear.\" While Bentley will need to invent a heritage for its SUV, which is likely to become the entry-level Bentley, Lamborghini has no such issues. It built its V12-powered, four-door LM002 from 1986 to 1993, and unlike Porsche\'s Cayenne, it was a hard-core off-roader evolved from a military prototype. Though a sales flop, Lamborghini believes it will lend credibility to a Raging Bull-branded SUV, which could appear as a concept car as early as next year. Sources insist Lamborghini sees a stronger market for an SUV than for a four-door, fast limousine, with the Italian company intending to pitch its high-rise five-door below its Gallardo replacement but well above current fast SUV contenders such as the forthcoming Mercedes-Benz M63 AMG, the BMW X5 M and X6 M and Maserati\'s Kubang. The move will spell the end of the line for fans of Lamborghini\'s Estoque concept four-door, which debuted at the 2008 Paris Motor Show but was left without an architectural home when it was decided to move the next Porsche Panamera onto a VW Group platform, rather than its current stand-alone base. While Porsche has broken with its historical roots to put a turbo-diesel engine in both the Panamera and the Cayenne SUV, Lamborghini insiders insist its SUV will never have a diesel engine, while Durheimer hinted not to expect a diesel in the Bentley SUV. At least, not initially. \"We will not make any experiments with it and I have declared 12-cylinder technology being a core of Bentley,\" he said. \"But I have no problem with diesels. Modern diesels are really powerful and their pickup behaviour is very impressive and they don\'t smell and they are not as noisy anymore. \"The Mulsanne with the V8 is the Bentley engine, but if you drive it compared to a V8 TDI, you probably would not feel it. \"In some markets, like the UK, France, Austria and Italy, diesel might be the entry ticket to sell a car, and if you can do it with diesel as an additional or add-on engine for us, why should it not be in the SUV?\"