Dense, freezing fog is causing travel disruption across the capital as London City Airport joins Heathrow in cancelling flights.
The Met Office reported that visibility at the Docklands hub was down to 100m. A Gatwick spokesman said there had been one cancellation so far but warned delays were likely.
Travellers have been advised to check the status of their flight before leaving home after around 100 flights were cancelled at Heathrow.
A yellow fog warning has been issued for rush-hour with severe weather affecting the entire south of the country, the south east the worst affected. Freezing temperatures and low winds might keep the dense fog in place until early afternoon, say forecasters.
Road users are also being warned of difficult driving conditions after freezing temperatures overnight. A low of –6.2C (21F) was recorded in Northolt, around 10 miles from Heathrow.
A message on the London City Airport website said foggy conditions were leading to cancellations and delays. About 15 flights had been cancelled by 6.45am with others delayed.
A Stansted Airport spokeswoman said there had not been any cancellations, but advised passengers to check with their airlines for the latest information and allow enough time to get to the airport.
The Thames Clippers and the Woolwich Ferry were operating normal services despite the poor visibility.
Met Office forecaster Mark Wilson said temperatures will struggle to top 0C (32F) in the worst affected areas on Monday, where fog is likely to persist.
Meanwhile, commuters were affected by a shortage of trains during the early peak with TfL advising people to use alternative routes. Faults also caused minor delays on the District line and Overground. Check the latest status at the TfL website .