Speaking with Autocar at the 2017 Los Angeles motor show, head of AMG, Tobias Moers, said the 53 branding will be stuck on AMG models using petrol-electric hybrid powertrains - in the process hinting that an E53 could follow the CLS.
According to the report, the CLS53 - and likely subsequent AMG 53 models - will feature a 3.0-litre turbocharged straight six matched with a 50kW electric motor.
The new CLS450 4Matic (pictured), meanwhile, features a 270kW/500Nm 3.0-litre turbo straight six with a 16kw/250Nm mild-hybrid system, helping it sprint from 0-100km/h in 4.8 seconds.
With the uprated electric motor, the AMG CLS53 is expected to develop around 429bhp (320kW), which, combined with its increased torque should give it acceleration equivalent to that of the outgoing 430kW/800Nm V8 AMG CLS63 - which hits triple figures from a standstill in a claimed 4.1 seconds.
This announcement confirms rumours that the new-generation CLS range would be headlined by a '53' hybrid AMG variant, though it's still unclear whether a V8-engined AMG 63 version will be offered.
Previous reports have speculated that the AMG 53 variant will leave room for the production version of the four-door 'AMG GT4' sedan, which has been spied on several occasions looking very much like a hotted-up CLS with different tail-lights.
The new model was previewed by the AMG GT concept at this year's Geneva motor show, which utilised its own hybrid setup - though it teamed the company's 4.0-litre twin-turbo V8 with an electric motor for a combined power output of "up to 600kW".
For now, we'll have to wait until the Detroit motor in January to see what the new AMG CLS53 is all about.