More than 400 flights were cancelled by Heathrow Airport after bosses were forced to close a runway to de-ice it, BBC News reports.
The northern runway was shut from about 10:45 GMT until midday for de-icing and snow-clearing. Warnings of travel disruption were issued ahead of the snow, which began falling in and around central London shortly after 08:00 GMT.
BBC meteorologist Peter Cockroft said much of London was likely to see up to 10cm (4ins) of snow. Snow will start to ease off on Saturday morning, Cockroft added.
Network Rail said 85% of trains in London and the South East had been running on time as the snow started falling. Some train operators were running revised services and drivers have been advised to allow extra time for journeys.
A Heathrow spokesman said the de-icing of the northern runway, which was expected to take around 45 minutes, meant there were delays and cancellations at the west London airport.
He added it was possible the southern runway might have to be closed for similar work to be done once the northern one was reopened.
The airport has also extended the length of time between flights for safety reasons.
"This is primarily because of problems at other airports," the spokesman said. "Obviously, the snow is affecting the whole country and it just means there's a knock-on effect here."
There are also some delays and cancellations at London City Airport, where the runway was closed for de-icing for an hour from 09:00 GMT.
A London City Airport spokesperson said: "We have plans in place to deal with snow."
Passengers travelling to Heathrow, London City and Gatwick airports have been advised to check the status of their flight before travelling to the airport.