Audi is preparing to take on the latest BMW M5 and soon-to-be-revised Mercedes E63 AMG with an all-new Audi RS6. Power will come from an uprated version of the new twin-turbo 4.0-litre V8 from the S6, S7 and S8.

Caught testing at the Nürburgring, the new RS6 is expected to make its public debut at next year’s Geneva show, with UK sales starting in mid-2013. It will be sold as both a saloon and an estate.

The new force-fed V8 replaces the naturally aspirated 5.0-litre V10 of its predecessor. Developed with Bentley, it kicks out a nominal 513bhp in the S8.

However, detailed changes, including added turbocharger boost pressure, are said to liberate a further 59bhp, taking it to 572bhp — the same output as the old V10 and well beyond the 414bhp of the S6 saloon and Avant. Torque is also set to climb beyond the 479lb ft of the S8. One Audi insider told Autocar that it’ll be more than 516lb ft.

The RS6’s power will eclipse both the 552bhp of the M5 and the maximum 549bhp available with the current E63 AMG.

Drive will be apportioned to all four wheels by Audi’s torque-sensing all-wheel drive system, complete with an active rear differential with torque vectoring. It’s so far unclear if the new car will come with an eight-speed torque converter automatic gearbox or a newer seven-speed, dual-clutch auto.

Greater use of aluminium and carbonfibre will help to reduce weight by up to 100kg over its predecessor.

Nothing is official, but expect 0-62mph acceleration in under 4.5sec and a top speed limited to 155mph — well short of the potential 198mph that Audi engineers claim.