Nissan says it has uncovered falsified data from car exhaust emissions tests at most of its Japanese factories.
The firm said emissions and fuel economy tests had "deviated from the prescribed testing environment".
The carmaker added that inspection reports had been "based on altered measurement values".
Nissan pledged there would be a "full and comprehensive investigation" and said "appropriate measures" would be taken to stop any future recurrence.
Nissan said it was looking into whether whether the issue affected vehicles exported from Japan to other markets.
The company said it had rechecked "reliable" data and confirmed that all vehicles except the GT-R sports car conformed to Japanese safety standards. The GT-R was not included because it had not been able to check enough vehicles as yet.
by Jonty Bloom, BBC business correspondent
This is a very embarrassing affair for Nissan and it will damage its reputation, but it does not seem at the highest levels to have been deliberately trying to beat the system.
VW was caught cheating emissions testing by deliberately writing software that meant its cars met emissions standards only when they were being tested but not at any other time.
Nissan on the other hand seems to have been running its testing system very badly, they did not meet legal requirements and measurements were altered.
That does not sound as bad as what happened at VW but it is still very shocking. This was going on at all of Nissan's factories in Japan, bar one; that means it is hardly a one-off accident or down to a few rotten apples.
Nissan is still investigating what went wrong.
But, if this scandal ends here, Nissan will probably be able to say it made a sin of omission rather than VW's sin of commission.
Nissan's shares fell more than 4.5% on Monday after the company alerted investors that a statement on exhaust emissions was imminent.
Last year, Nissan recalled 1.2 million vehicles in Japan after regulators said safety checks did not meet domestic requirements.
A subsequent investigation into why its safety inspections did not meet government standards has now led to the latest revelations.
The admission by Nissan comes after a huge scandal involving diesel emissions test cheating by Germany's Volkswagen.
Last month, VW was fined €1bn (£880m) by German prosecutors for selling more than 10 million cars between mid-2007 and 2015 that had test-cheating software installed.