In the case of BMW, it submitted the vehicle test results from the X6M model for its X5M, the ministry said.

The local unit of the German automaker has explained some of the X6M's test results were included as the two models have the same engine and are under the same certification number, according to the ministry.

BMW Korea reportedly maintains that the cars should be allowed to share their test results and plans to reiterate such a position at an upcoming government hearing.

The ministry earlier said it has delivered notices for hearing schedules to the three carmakers, and that administrative measures will be taken in mid-December if necessary.

The local unit of Porsche is accused of modifying emissions test results of its three vehicle models.

The ministry said the company voluntarily reported the defects in its certification documents to the ministry and the prosecution over the course of their investigation.

The company said it will continue to cooperate with the investigation, adding it has voluntarily suspended sales of the three vehicles models as of Monday.

Once the irregularities are confirmed, the government will nullify the certifications and ban sales of the six models, the ministry said. The companies can be fined up to 6.5 billion won (US$5.56 million) in total.

Market observers noted the impact of a sales ban may still be limited as the vehicles facing such a penalty only make up a fraction of imported cars sold here.

In the first 10 months of the year, 185,801 foreign vehicles were sold here, according to the Korea Automobile Importers and Distributors Association (KAIDA). The 10 vehicle models sold a combined total of 4,349 units over the cited period, accounting for 2.3 percent of the total.

"It will not be a matter of how many cars they sell, but an issue related to the image and reputation of their brands," a market observer said, asking not to be identified.

The ministry said the cars will not be subject to a recall order, as the issue does not involve vehicle defects.

Hong Dong-gon, a ministry official, said the irregularities seems to have been committed to speed up the administrative process and expedite the local launching of new cars.

"Errors in certification documents are in violation of the Clean Air Conservation Act, regardless of intention," he said. "We will now annually check the documents of not only the vehicles seeking approval, but also those who have already won approval." (Yonhap)