More design sketches of the new Mercedes Concept S-class Coupé have leaked online, ahead of the car's debut at the Frankfurt motor show next week.

So far Mercedes-Benz has only confirmed that one, originally released on its Facebook page, is official.

The car, which had previously only been seen in disguised form and undergoing testing, will form part of a five-strong line-up for the new S-class and rival the Bentley Continental GT.

Mercedes is aiming for a sleeker, more muscular appearance for the S-class Coupé, eschewing the two-door saloon style of the CL it replaces.

Latest spy shots show the S-class with a narrow glasshouse and a dramatic swage line that runs from the front wing to the rear wheelarch.

The rear of the S-class Coupé features LED lights and a unique design that appears unrelated to any other Mercedes models, including the new S-class saloon.

A gentle sloping of the rear windscreen implies rear seat accommodation will remain a priority for the two-door car. Prominent exhaust pipes also hint at the more sporting intent of the coupé.

Further additions to the line-up will include a convertible based on the Ocean Drive concept and an extra-long-wheelbase version under the 'Pullman' name.

Mercedes-Benz's new coupé is expected to arrive in 2015, following a debut for the production car at next year's Geneva motor show.