Very high levels of pollution are expected in some areas of south east England due to the warm conditions creating a build-up of local pollutants from sources such as traffic fumes, combined with pollution from the continent and Saharan dust from the south.
Met Office forecaster Dan Williams said: “Friday is expected to be the warmest day with 69.8F (21C) or 71.6F (22C) possible.
“Air is coming from the tropics and passing over land to the south of the UK before coming here.”
Much of the South East and eastern England will see high levels of pollution, although the problem is expected to be short-lived, with Atlantic winds dispersing the polluted air in the early hours of Saturday.
A spokesperson for the Met Office said: "There will be a little bit of Saharan dust in the air.
Most people will not notice it unless it rains and is washed out of the air. If this happens then people may seen fine particles of dust on their car windscreen for example."
In areas experiencing very high levels of air pollution, adults and children with lung problems, adults with heart problems and older people are advised to avoid strenuous activity.
A Defra spokeswoman said: "Locally generated air pollution, combined with pollution from the continent and Saharan dust, could cause high or very high levels on Friday.
"This is expected to clear on Saturday and pollution levels will return to low throughout the morning."
People are being advised to reduce physical exertion, particularly outside, and asthma sufferers may need to use their reliever inhaler more often.
Dr Penny Woods, chief executive of the British Lung Foundation, said: "This latest high air pollution episode, coming so soon after the last, is troubling, and could again put people living with respiratory conditions at risk of worsening symptoms."
She advised people who find their health affected to take simple steps including avoiding busy roads and refraining from strenuous exercise, while those who carry a reliever inhaler should always have it with them.
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