A Supreme court ruling could mean the Government has to speed up its plans to deal with air pollution, just days after London was blanketed in smog.
The Government has been fighting legal action for four years over its failure to meet EU nitrogen dioxide levels.
High levels of air pollution are to hit central London today.
The warm weather is pulling hot air in from France, bringing dense smog with it.
Older people and those with asthma or lung problems are being urged to take extra care.
Londoners have basked in the hottest day of the year so far today. Temperatures rose to 26.3C at Heathrow Airport. That compares to around 22C in Ibiza.
Gravesend in Kent saw a high of 26C and there were a few other places close to that around the South East.
London Ambulance Service experienced their third busiest weekend ever with the warm weather and the FA Cup final contributing to the rise in calls to the service. Temperatures are expected to cool off slightly tomorrow.
With 'moderate' or 'high' levels of air pollution forecast for London today, here's the advice from campaign group, Clean Air in London.
Adults and children with lung problems, and adults with heart problems, who experience symptoms, should consider reducing strenuous physical activity, particularly outdoors.
Adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion particularly outdoors and particularly if they experience symptoms. People with asthma may find they need to use their inhaler more often. Older people should reduce physical exertion.
Another smog alert has been issued for London on what could be the hottest day of the year so far.
Researchers from King's College London say air from parts of Europe will move across the capital, bringing with it 'industrial and urban emissions'.