The environment department said air pollution in some areas reached the top rung on its 10-point scale.
The department said the smog was caused by pollution from Britain and industrialized areas of the continent - trapped in place because of light winds - mixing with dust blown up from a storm in the Sahara desert.
Many motorists across England awoke this week to find cars covered in a film of red dust left by overnight rain.
Paul Cosford of Public Health England told the BBC that people with heart or respiratory problems should "reduce the amount of strenuous exercise outdoors over the next few days."
Experts said Britain's smog was the result of an unusual combination of factors. Helen Dacre, a meteorologist at the University of Reading, said conditions had conspired "to create a 'perfect storm' for air pollution."
"Toxic gases, such as nitrogen dioxide and ozone, as well as fine dust particles in the air blown in from the Sahara and from burning fossil fuels, all contribute to cause problems for people with heart, lung and breathing problems, such as asthma," she said.
Despite efforts to make industry and automobiles cleaner, air pollution remains a major problem in Britain and across Europe.
Last month, Paris took the drastic step of banning half the city's cars from the roads for a day after a week in which pollution levels exceeded those in notoriously smoggy cities including Beijing and Delhi.
The World Health Organization said last week that air pollution is the world's single biggest environmental health risk, responsible for about one in eight deaths.
The WHO said air pollution kills about 7 million people worldwide every year, with more than half of the fatalities due to fumes from indoor stoves and the rest from outdoor pollutants.
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