The reduced visibility gave the major airport no choice but to stop 100 out of its 1,300 scheduled flights, a spokesman said.
It marks another day of misery for travellers after 100 flights were cancelled at Heathrow on Monday. Gatwick, London City, Stanstead and Southampton airports were also hit.
A Heathrow spokesman said: “Persistent freezing fog across the south east has reduced visibility at Heathrow again today.
“With Heathrow operating at more than 99 per cent capacity, there are no gaps in the schedule that can be used for delayed flights and 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. As always, Heathrow’s top priority is the safety of passengers and we apologise to those whose travel has been affected by today’s weather.”
Freezing fog develops across the Thames
The Met Office said a “severe” warning was in place covering England, except for the North East and south-east Wales, which will remain in place until 11am on Tuesday.
Met Office spokesman Grahame Madge said: "We are expecting these conditions to last a for a few days to come.”
FOG causes DISRUPTION across Southern England
Mon, January 23, 2017
Dense fog has caused travel disruption across southern England, with thousands of air passengers facing cancellations and delays.
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A London bus makes its way over Westminster Bridge in London
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A Gatwick spokesman also offered advice to passengers and said: "Air traffic control restrictions imposed due to fog across the South East and Europe may cause some delays to flights."
Edinburgh Airport told passengers to contact their airlines after flights towards the south of the UK were hit on Monday.
Bristol Airport is operation as normal and Stansted has not yet cancelled any flights.
Amid the warnings of freezing fog conditions, the Department for Environment, Food and Rural Affairs said 'very high' pollution levels will persist in parts of Britain today.
Some regions will be hit by 'high' levels again on Wednesday before they drop to 'low' by the weekend.
Delays at London City Airport on Monday
Gatwick, London City, Stansted and Southampton airports all experienced heavy delays to flights yesterday.
Edinburgh Airport advised passengers to contact their airlines, after flights towards the southern Britain were affected on Monday.
UK covered in complete mist yesterday as 100 flights cancelled
Bristol Airport said flights were operating as normal while Stansted said it had no cancellations yet, but urged passengers to contact their airlines for the latest information.