This is the shocking moment a packed passenger jet appears to fill with smoke as a result of warm air from outside being pumped into the cabin.

The video was recorded on a flight between the cities of Yangzhou and Changchun in China on August 18.

Fliers said that the air coming in was incredibly cold, and passengers had grabbed blankets to try to avoid the worst effects of the low temperature.

Experts added that although it looked frightening, it was completely harmless and surprisingly common in flights when warm, humid air from outside was sucked into the cabin via the air conditioning that rapidly cooled it.

They said that if the air had high humidity, this caused condensation, which would have the effect of clouds of vapour being pumped into the cabin, making it seem like thick fog or smoke.

However, they said it was otherwise harmless.

The passenger who recorded the footage joked: 'This was not a plane, it was a flying freezer.'

One online commentator said it was like a fairy story where fog suddenly descended quickly.