Previous S-Classes have had Pullman variants, but they’ve never been officially available in Britain.
The super-stretched model will top the new S-Class range, and is expected to go on sale in around 2015.
A slightly shorter model, aiming to fill the gap left by the now discontinued Maybach brand, will also be introduced.
Mercedes boss Dieter Zetsche has said the new Pullman model will be the firm's Rolls Royce Phantom rival.
Mercedes also confirmed that it will launch convertible and coupé versions of the S-Class. The latter arrives first, and will be available in the UK from summer next year. It will debut a nine-speed automatic gearbox – and a head-up display.