The warning comes on what could be the hottest April day recorded in Britain in nearly 70 years, breaking the 1949 record if 28C is recorded anywhere in the country.
But the Department for Environment, Food & Rural Affairs (DEFRA) have put a dampener on the sizzling heat with the warning of the 'toxic plume', which is heading to the UK on warm air said to be from France and Spain.
The pollution is reported to be at its worst today (Thursday) and tomorrow, with people who suffer from lung and heart problems advised to avoid strenuous outdoor activities and to stay indoors if they feel discomfort.
The east coast of England will be one of the worst hit areas, with pockets of pollution in southern and eastern areas.
Leeds is expected to reach around 23C today, making it the hottest day of the year by some way so far.