London smog alert

A major smog alert was issued in London today.

The Government said the smog, caused by pollution and high temperatures, could affect the health of thousands of Londoners.

Asthmatics and others with respiratory problems were told not to exercise today for fear of triggering an attack. Experts said up to 15 per cent of Londoners could be at risk.

Professor Martyn Partride, of the National Heart and Lung Institute, said the current conditions were extremely worrying. "I have no doubt admissions for asthma-related illnesses will rise as a result of this smog," he said.

"People also need to be particularly careful if there is a thunderstorm while it is still about - that's when it seems to affect them most."

Ben Bradshaw, minister for local environmental quality, urged everyone "sensitive to air pollution, such as those with heart and lung disease" to be aware of the heightened risk to their health and avoiding exertion outdoors.

The warning is a reminder to thousands of Londoners who remember the smogs of the 1950s and 1960s before "clean air" legislation.

High ozone levels are forecast tomorrow for London, the South-East, East Anglia and central England. They are likely to persist until Saturday.

Professor Frank Kelly, of Kings College's pollution-department, said many Londoners would feel the change in conditions as "a tightening in the chest".

With the Met Office predicting a hot summer, it is feared smog levels could mimic those of the 2003 heatwave.

It was then estimated that "up to 800 premature deaths may have occurred as a result of the poor air quality," Mr Bradshaw said.

Today's warning is the first for London since 2003, Defra officials confirmed. Summer smog is produced by sun acting on substances in the lower atmosphere such as car fumes and solvents, producing groundlevel ozone.

Smog can also contain elevated levels of nitrogen dioxide and breathable dust, known as particulate matter.

Today's warning also raised the political temperature in the capital, with the Liberal Democract blaming the Mayor for failing to tackle pollution .

Mike Tuffrey, Lib-Dem London Assembly environment spokesman, said " six years of rhetoric ... wear thin when the problems are getting worse." But Ken Livingstone hit back, saying his transport policies have "reduced emissions across the whole of London".

London's most famous smog was in December

1892. It lasted for three days and resulted in around 1,000 deaths.

It was followed by several major smog incidents in the 1950s and 60s, when the phrase "pea souper" was coined.