Experts said people suffering from lung diseases including asthma, particularly the elderly, should be aware that their breathing could worsen.
Ground level ozone is formed when sunlight acts on nitrogen dioxides and other atmospheric substances which come from a range of sources, including petrol and other fuels.
Particulates, or PM10s, also come from sources such as vehicles.
The smog alert, the first for more than a year, comes as monitoring in London revealed that one site, on the Marylebone Road, has exceeded EU rules for the number of days in the year in which high levels of PM10s are permitted.
The UK will not face fines for the breach, as the EU has given Britain an extension which means it has until June before it has to start meeting the standards in the capital.
But Green Party London Assembly member Jenny Jones said London's residents and visitors should be made aware of the high pollution levels ahead of the royal wedding.