The first "very high" pollution alert has been issued for London by mayor Sadiq Khan, under a new system for warning people about poor air quality.
Warnings have been issued at bus stops, Tube stations and roadsides in the capital, and Mr Khan said everyone from the vulnerable to the physically fit may need to take protections to protect themselves from the "filthy air".
The warning came as swathes of the UK suffered very high or high levels of air pollution from pollutants known as particulate matter or PMs, coming from sources including traffic emissions which are failing to disperse in the still weather.
Most English regions, South Wales and Northern Ireland experienced very high or high levels of the pollutants on Monday, data from the Environment Department's (Defra) UK Air website showed.
Air pollution from sources including factories and vehicles, particularly diesel engines, is linked to the early deaths of around 40,000 people a year in the UK - and causes problems such as heart and lung diseases and asthma.
In very high pollution conditions, people are advised to limit exercise outside - while those with lung and heart problems and older people should avoid strenuous activity.
Where there is high air pollution, adults and children with lung problems and adults with heart problems, as well as older people, should reduce the amount of strenuous exercise they do.
High and very high levels of pollution can cause people with asthma to need to use their inhaler more.
In the capital, the London Air Quality Network run by King's College, London, said the cold, calm and settled conditions over the weekend caused a build-up of local emissions from traffic and wood burning, combined with pollution from the continent.
The conditions prompted a "black" alert for very high levels of particulates - 10 out of 10 on the air pollution scale - with problems expected to continue into Tuesday.
Mr Khan said: "Today the shameful state of London's toxic air has meant that I am forced to trigger the first 'very high' air pollution alert under my new comprehensive alert system.
"This is the highest level of alert and everyone - from the most vulnerable to the physically fit - may need to take precautions to protect themselves from the filthy air.
"Safeguarding the health of Londoners from pollution is my number one priority - and we are implementing the boldest policies of any city in the world to get to grips with the awful problems we inherited."
He said measures were being brought in to clean up the bus fleet, charging for the dirtiest toxic diesels, which are a significant source of air pollution, and extending the "ultra low emission zone".
But he called on the Government to take action, including introducing a national diesel scrappage scheme, reform vehicle excise duty and bring in a new Clean Air Act to tackle the problem.
Defra forecasts that many places across England and Wales will continue to experience high air pollution on Tuesday due to continuing light winds, while very high levels could be seen again in south east, central and eastern England.
Dr Yvonne Doyle, London regional director for Public Health England and statutory health adviser to the mayor of London, warned that people who suffer lung and heart conditions, as well as older people, should avoid "strenuous" activity in high pollution areas.
She said: "Some areas of the country including London have recorded very high levels of ambient air pollution. PHE is advising people in those areas to reduce physical exertion, particularly when outdoors and especially if they experience symptoms such as a cough or sore throat."