To date, the carmaker has assessed its hybrids' fuel economy in line with EPA guidelines, but has faced complaints and a lawsuit that the official figures were not realistic in real-world use.

It will now begin voluntarily testing future models separately and, as a result, the C-MAX Hybrid’s official fuel economy will drop from 47mpg (56.4mpg Imperial) to 43mpg (51.6mpg Imperial).

The company said variations in fuel economy were greater for hybrids than for conventionally-powered models, because small differences in driver behavior could lead to the petrol engine being used more frequently.

Existing customers will be sent a goodwill payment to cover the difference in fuel economy resulting from the new test. Those who purchased the car outright will get $550 (£352), while leasing customers will get $325 (£208).

Raj Nair, group vice president, global product development, said: ‘Ford is absolutely committed to being a leader in the hybrid market and to top fuel efficiency across our lineup. We are taking actions with our popular C-MAX Hybrid so that customers are even more satisfied with the vehicle’s on-road fuel efficiency performance.’

The C-MAX Hybrid is being updated for the 2014 model year, with software updates, aerodynamic improvements and reduced friction engine oil all aimed at improving real-world fuel efficiency. As yet, the model has not been assessed using Ford’s new economy test.