Londoners have been warned to expect higher levels of pollution as a Saharan dust cloud arrives in the capital amid soaring temperatures.
Forecasters predicted that the mercury could reach a sweltering 26C in London and the south-east this weekend, making it hotter than Istanbul and Ibiza.
But the warm weather has also triggered a smog alert and health warnings across the south-east.
Defra, which issues official pollution warnings, has urged those with lung or heart problems to avoid strenuous, outdoor activity.
Asthma sufferers have also been advised that they may find the need to the use their reliever inhaler more often.
The dust cloud, formed from the Saharan desert, is expected to affect much of England and Wales for the next four days.
The Defra forecast said “moderate” air pollution is expected to persist from Thursday to Sunday.
A Defra spokesman said: “Strong south-easterly winds blowing in air from the continent could lead to moderate levels of pollution on Thursday and over the weekend in parts of England and Wales.
“Levels are expected to fall after the weekend.
“This does tend to happen during the change in the temperature around spring time.”