Smog alert for England and Wales
The government has issued a "smog alert" in England and Wales for ozone and polluting particles known as PM10s, which can affect people's health.
The Department for Environment, Food and Rural Affairs said high levels of pollution were likely from Thursday and would continue over the Easter weekend.
It said the alert was due to warm and still conditions brought on by a high pressure system.
Defra is urging the public to take "sensible precautions".
It said some people, including those who have asthma - and particularly the elderly - could be affected by the high pollution levels and may notice an impact on their breathing.
It advised people to avoid taking exercise outside in the afternoon if they are susceptible to the pollutants and not to take unnecessary short car journeys, to help reduce the amount of pollutants from exhausts.
Defra said its freephone helpline - 0800 556677 - would give regular updates on the levels of particles, and also offer health advice to those who may be particularly sensitive to air pollution.
Cher Piddock, an Asthma UK adviceline nurse, said the smog warning was a "timely reminder" that the combination of warm weather and pollution could pose health risks.
"Around two-thirds of people with asthma say pollution triggers their condition, so Asthma UK recommends that people who have pollution as a trigger avoid going out if air quality is poor," she said.
"We also recommend that people always carry their inhaler, avoid exercising outdoors on hot days, especially in the afternoon, and keep windows shut whenever possible."
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, are also emitted by car exhausts.
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.
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.
"We have the prospect of hundreds of thousands of visitors flocking to central London, to stand around all day in one of the most beautiful, but polluted cities in Europe," she said.
"The mayor should be making people aware of what they are breathing in and the risks to their health."
The smog alert comes as the great getaway begins, with Britons enjoying a run of two double bank holiday weekends.
This year's late Easter and the royal wedding, on 29 April, have resulted in some workers only having to take only three days' leave off next week to get an 11-day break.
Engineering works will affect some rail routes, and major motorways - including a now fully reopened M1 - were expected to become very busy from Thursday lunchtime.
About 18m people are expected to use their cars, but the AA predicted high fuel prices will mean people will make shorter journeys.
The warm weather is set to continue through the Easter weekend for most of the UK, especially in England and Wales.