Scotland’s First Minister Nicola Sturgeon was one of those hit by the disruption.
She had been due to attend an engagement in London this morning but was delayed at Glasgow Airport because a small number of flights south were cancelled.
Flights from Edinburgh and Aberdeen Airports were also affected by the freezing fog in London.
Dense fog is causing travel disruption across southern England, with thousands of air passengers facing cancellations and delays.
Travellers were advised to check their flight’s status before leaving home after around 100 flights were cancelled at Heathrow, while there were further cancellations at London City Airport.
The Met Office has issued a severe weather warning for London, south-east England and parts of south-west England over dense fog that is likely to hit the morning rush hour.
Heathrow Airport warned passengers to check their flight details before they leave home as visibility at the hub was reduced to 100 metres.
A yellow warning of fog is in place until 10.30am, but freezing temperatures and low winds could see it persist into the afternoon, Met Office forecaster Mark Wilson said.
“Heathrow is currently on 100 metres’ visibility, which is very dense fog, Gatwick is on 100 metres, London City Airport is on 100 metres and Stansted is on 100 metres,” he said.
“It looks like it will generally improve through the last part of the morning and into the afternoon, but one or two spots will hang on to fog for much of the rest of today.”
Road users are also being warned of difficult driving conditions after freezing temperatures overnight.
Temperatures dipped to their lowest in Katesbridge, Northern Ireland, where minus 6.9C (20F) was recorded.
In England a low of minus 6.2C (21F) was recorded in Northolt, around 10 miles from Heathrow.
Around 100 out of 1,300 flights were cancelled as reduced visibility affected the tight take-off and landing schedule at Heathrow.
A spokesman for the airport said: “Foggy weather across the South East has reduced visibility at Heathrow.
“As a result, some passengers may experience disruption to their journeys today.
“Passengers should check their flight status with their airline before coming to the airport.
“Heathrow’s top priority is the safety of passengers and we apologise to those whose travel has been affected.”
A message on the London City Airport website said foggy conditions were leading to cancellations and delays.
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.
Mr Wilson said temperatures will struggle to top 0C (32F) in the worst affected areas on Monday, where fog is likely to persist.
Elsewhere, highs of between 3C (37F) and 5C (41F) are expected.