An issue with the test process has forced Kia and Hyundai to adjust the range expected from their electric SUVs.

The long-range Hyundai Kona EV has had its range downgraded on the WLTP test cycle, after the company discovered its external test agency hadn't followed the correct test procedure.

Kia last week downgraded the range of its e-Niro for the same reason. The re-rating means the 39kWh Kona is rated for 289km instead of 300km, while the 64kWh model is officially good for 449km rather than the 470km initially promised.

Meanwhile, the e-Niro is now rated to 455km and 289km with the 64kWh and 39kWh battery packs respectively.

“In testing the Kona Electric to establish its homologated electric vehicle driving range, the independent organisation overseeing the process accidentally provided an incorrect testing methodology and then approved the results it generated," Hyundai Europe said in a media release.

“This led to the Kona Electric being tested for a disproportionate length of time on the WLTP ‘urban’ cycle – comprising lower overall vehicle speeds and a reduced energy requirement – resulting in an overestimation of the vehicle’s all-electric range.”

According to the company, the issue was discovered because of its "ongoing homologation work". How the issue occurred in the first place is being investigated at the moment.