A police officer based in Ulsan said that the three people who died included two Hyundai researchers involved in car testing at Hyundai’s No 4 factory in Ulsan at around 3pm.

The officer told Reuters that police and the labour ministry were investigating the incident.

The three were found unconscious in a car while they were testing the car in a “chamber”, according to a union official.

Media reports said the three had suffocated.

A company spokesman did not have an immediate comment.

The Ulsan plant is Hyundai’s biggest manufacturing facility, with its own port and an annual production capacity of 1.4 million vehicles, including exports of 1.1 million units.

In November last year, Hyundai Motor broke ground on a 2 trillion won (US$1.44 billion) plant in Ulsan dedicated to making electric vehicles in South Korea, as the automaker accelerates a shift away from petrol-powered cars.