Most of the flights grounded were heading to destinations in Britain or Europe.
A Heathrow spokeswoman told AFP: "We're looking at 190 cancellations at the moment. They're mostly short-haul -- a mixture of flights to mainland Europe and domestic flights."
The conditions also caused flights arriving at Heathrow to be delayed by an average of one hour, while departing flights were subject to similar delays.
"We expect the fog to last into the evening," the spokeswoman said.
The fog, which was especially thick in southern England, also led to the cancellation of every early morning service at London City, an airport mainly used by business travellers.