For the third day out of four, airline passengers flying in and out of South East England face cancellations, delays and diversions.
Fifteen Ryanair flights heading for Stansted were diverted after a private jet damaged landing equipment. Arrivals from Spain, Italy, Eastern Europe and Scandinavia were switched to five alternative airports, from Gatwick to East Midlands.
The rival airline, easyJet, diverted domestic flights to Luton and Birmingham. Passengers were told: "We are pro-actively utilising our standby aircraft at London Stansted to mitigate delays as best we can for the remainder of the day."
Stansted airport issued a statement saying: "Earlier this morning an arriving Gulfstream G550 was involved in an incident that has resulted in some damage to our ILS (instrument landing system). The aircraft landed safely. As a result of this incident and low visibility there have been restrictions on arriving flights."
On a normal day, the damage would not have caused undue problems. But it led to the temporary downgrade of the airfield to Category I. As a result, instrument landings were suspended - and because of fog, aircraft could not land visually.
While departing flights initially left on schedule, subsequent services were cancelled or seriously delayed. Disruption at Britain's main low-cost airport is likely to continue for the rest of the day.
Flights to and from London City airport were cancelled or diverted because of fog, with British Airways passengers from New York ending their transatlantic journey in Gatwick rather than Docklands.
At the UK's busiest airport, Heathrow, BA cancelled 20 domestic and European departures and arrivals as delays built up. Poor visibility at Heathrow reduced the "flow rate" of aircraft movements.
A queue for de-icing at Gatwick, easyJet's main base, caused delays to the first wave of aircraft departures, which rippled through later operations. Many flights were delayed by an hour or more, and Flybe's flight from and to the Isle of Man was cancelled.
Flybe, the UK's biggest regional airline, has hit out at the air-traffic control firm, NATS. On Saturday three-quarters of Flybe services were more than 15 minutes late because of an IT failure at the main control centre. The airline's Chief Commercial Officer, Paul Simmons, said: "That is totally unacceptable and meant thousands of our passengers had their travel badly disrupted."
"It's clearly not a fair system when NATS can wash their hands of any financial compensation to the millions of passengers who were let down. But the airlines once again have to act as 'the Insurers of last resort' and pick up the tab."
Sir Howard Davies' Airport Commission is due to make its interim recommendations next week on tacking the aviation capacity crunch in South East England.