Saharan dust blowing in from North Africa shrouded skyscrapers in London's financial district, coated residents' cars and pushed pollution to high levels yesterday.
The city's pollution index reached seven on a scale of 1-10, meaning those with lung and heart problems should reduce exercise, according to government recommendations.
London faced "high" pollution levels, the state Met Office forecaster said.
Cars outside Prime Minister David Cameron's Downing Street residence were covered with dust blown in from the desert 3,320 kilometres to the south, and the offices of some of the world's biggest banks faded into the haze in east London's Canary Wharf. Dust was thrown up by sandstorms in northwest Africa at the end of March and deposited in rains, the Met Office said in a blog.
"Weather conditions have conspired to create a perfect storm for air pollution," Helen Dacre, a meteorologist at the University of Reading, said.
"British car drivers and heavy industry create bad enough smog on their own, but the weather is also importing pollution."
Britain has struggled to reduce pollution levels and the European Commission in February threatened to sue the country for failing to meet limits on nitrogen dioxide, which can cause respiratory problems. The environment ministry has said London would not get its levels of the pollutant down to those limits until 2025.
"Most of this week we've had very light winds which has allowed local pollutants to build up," Laura Young, a Met Office spokeswoman, said. "The little wind we have had has been coming from east and southeast, bringing particles from across the [English] Channel. Add to that some Sahara dust."
Levels were forecast to abate today to a level of four in London, at the lower end of "moderate" pollution, and three or "light" on Friday, according to the Met Office.