Freezing fog has forced Heathrow to cancel up to 100 flights again today.
The reduced visibility means that planes landing or taking off every 45 seconds is impossible.
Gatwick says its passengers may experience some delays also.
Irish passengers are experiencing the knock-on effects of the bad weather here. Anyone traveling today is being told to check with their airline first.
The airport released this statement earlier.
Fog across London is expected to cause disruption to flights at Heathrow today. Please check with your airline prior to travelling. pic.twitter.com/kaTMbVnYSm— Heathrow Airport (@HeathrowAirport) January 24, 2017