LONDON • A cloud of freezing smog forced the cancellation of around 100 flights at London's Heathrow airport yesterday, as the city authorities issued a "black" alert for poor air quality in the British capital.

The foggy conditions affected at least 100 out of 1,300 scheduled flights at the hub - Europe's busiest in terms of passenger numbers.

"Persistent freezing fog across the south-east has reduced visibility at Heathrow again today," a Heathrow spokesman said.

He added that more flights were expected to be cancelled and apologised to passengers who may be affected by the weather.

"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."

The BBC reported cancellations and delays at London City, Gatwick, Southampton, Stansted and Edinburgh airports as well.

Flights to Amsterdam were the worst affected due to fog at the Netherlands' Schiphol Airport.

Ferries were suspended between Lymington, Hampshire, and Yarmouth on the Isle of Wight.

Britain's Met Office forecasters had a "severe" warning in place over all of England, a day after London issued its "very high" pollution alert.

A Met Office issued a fog and ice warning for Wales and most of England.

London Mayor Sadiq Khan said everyone from the vulnerable to the physically fit may need to protect themselves from the "filthy air". Warnings were issued at bus stops, Tube stations and roadsides.

The London Air Quality Network run by King's College said the cold, calm and settled conditions at the weekend caused a build-up of local emissions from traffic and wood burning, combined with pollution from the continent.

The conditions prompted a "black" alert on Monday for very high levels of particulates - 10 out of 10 on the air pollution scale.

AGENCE FRANCE-PRESSE