Mitsubishi Motors Corp. developed an illegal computer program for obtaining data to calculate fuel efficiency using a different measurement system than that required by the government, according to sources.

This is the latest finding in the ongoing scandal over the automaker's falsification of fuel efficiency data.

MMC developed the illegal program just after the government stipulated a measurement method in 1991, and MMC officials in charge used the program for many years.

This indicates that the MMC officials have been aware that using the program was illegal since the time the system was introduced.

"This is extremely serious. We will deal with it strictly," a senior official of the Land, Infrastructure, Transport and Tourism Ministry said.

The method introduced by the government is called the coasting test method. Each automaker runs its cars on test courses and measures their running resistance to obtain data for calculating fuel efficiency figures.

But MMC used a different method called the high-speed coasting test method.

After the scandal surfaced in April this year, MMC began internal investigations. According to sources, the high-speed coasting test method was used by MMC before 1991 for test runs of car models still in development.

To make data obtained by high-speed coasting appear to have been compiled through the government-designated method, MMC developed the program and used it for 25 years.

The program has systems that output numerical figures like those measured under the government-designated coasting test method, after data obtained with the high-speed coasting test method were input.

The sources said the program was for saving time for measuring again with the government-designated coasting test method during limited lengths of periods for developments.

The scheme was shared only by officials in the company's development divisions, and had not been reported to the management officials.

Some MMC employees insisted before the problem surfaced that it was wrong to use the high-speed coasting test method, but their opinions were ignored, the sources said.

The ministry learned the facts through its on-the-spot inspections, and will consider how to penalize MMC.