South Korea's government said earlier it plans to fine the Japanese company 330 million won (£195,000; $279,920) for manipulating emissions tests.
It plans a recall and to sue the head of Nissan's South Korean operations.
It is the first time Nissan has been officially accused of cheating. It denies the accusation.
Nissan has been tested by the European Union for emissions standards, but it concluded that Nissan vehicles used no illegal devices. The company said the South Korean authorities' findings differed from those of the EU.
Nissan is the second car manufacturer to have been accused of using emissions-cheating software.
In September, Volkswagen admitted cheating emissions tests in the US across its range of models affecting up to 11 million cars.
And last month, Japanese car manufacturer Mitsubishi Motors admitted it falsified fuel efficiency test data for decades.
Nissan is set to become Mitsubishi's biggest shareholder after agreeing last week to buy a 34% stake in the company.
South Korean's environment ministry probed 20 diesel car models in the wake of the Volkswagen emissions scandal. Only the Nissan Qashqai failed the test.
Hong Dong-kon, a director at the ministry, said the SUV's emission reduction device stopped operating when the engine's temperature reached 35 Celsius, about 30 minutes after the engine began to work.
"Usually, some cars turn off the emission reduction device when the temperature reaches 50 degrees Celsius, to prevent the engine from overheating. The Qashqai was the only vehicle that turned it off at 35 degrees,'' Mr Hong said.
When the emission reduction device stopped working, the Nissan vehicle's level of emissions was about the same or slightly higher than that of diesel cars of Volkswagen, which was fined for cheating on emissions tests.
"All auto experts expressed the opinion that it was clearly a manipulation of the emissions reduction device,'" Mr Hong added.
In a statement, Nissan said: "Nissan does not manipulate data related to our vehicles. The Nissan Qashqai has been correctly homologated under Korean regulations. Nissan has not and does not employ illegal defeat or cheat devices in any of the cars that we make.
"Furthermore, following stringent testing and using similar standards to the Korean tests, EU authorities have concluded that Nissan vehicles they tested used no illegal defeat device.
"Although the conclusions reached by the Korean authorities are inconsistent with those of other regulators, Nissan will carefully assess and consider appropriate next steps.
"Nissan is committed to upholding the law and meeting or exceeding regulations in every market where we operate. We are continuing to work with the Korean authorities."
More than 2 million cars have rolled of the assembly line at the Sunderland plant since Nissan began production of the car in 2006.
It is one of Europe's best selling cars and in February Nissan celebrated the fact that it had built more Qashqais than any other car in its three decades of manufacturing in Europe.