Smog Cloud Forces Flight Cancellations in London

A cloud of freezing smog forced the cancellation of around 100 flights at London Heathrow airport on Tuesday, as city authorities issued a "black" alert about air quality in the British capital.

The foggy conditions affected some 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.

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.

London mayor Sadiq Khan said everyone from the vulnerable to the physically fit may need to protect themselves from the "filthy air".

The London Air Quality Network run by King's College said the cold, calm and settled conditions over 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.