A toxic air pollution cloud blew over Britain on Thursday causing potential health problems for vulnerable people as the country struggles to meet its clean air targets.
Smog blowing in from Europe has mixed with home-grown pollution to send air pollution levels soaring to nine on a scale of 10 in some parts of the country.
Officials warned the pollution could trigger health problems for the elderly and those with respiratory illnesses, but said it does not pose a threat for the wider population.
A spokesman for the environment ministry (Defra) said: "Winds bringing in pollution from the continent, combined with locally generated pollution and still weather conditions, has led to some high pollution measurements across the UK.
Pollution levels are likely to remain moderate to high across Wales and southern England on Friday, before dropping to mainly low levels around the whole of the UK on Saturday, Defra said.
Mayor of London Boris Johnson issued a "high" air pollution warning earlier this week for the first time since he launched a pollution awareness campaign in January.
His website advised adults and children with lung problems and adults with heart problems to reduce "strenuous physical exertion", particularly outdoors.
Green Party politician Jenny Jones said the mayor's "Breathe Better Together" campaign was "disappointing and inadequate compared to the scale of London's air pollution challenge".
In January, campaign group Clean Air in London said the capital's busy shopping hub Oxford Street had breached the legal nitrogen dioxide (NO2) limit set by the EU for the whole of 2015.
Last year, the European Commission launched legal proceedings against Britain for failing to deal with air pollution, saying NO2 levels were "excessive" in many cities.
Britain was supposed to meet EU air pollution limits by 2010 but government projections indicate that some cities, including London and Britain's second city Birmingham, will not reach the target until 2030.
The Mayor of London's office says authorities are working to improve air quality in London by reducing pollution from public buses, investing in cycling and making homes and public buildings more energy efficient.
The Defra spokeswoman added: "Air quality has improved significantly in recent decades and we are investing heavily in measures across government to continue this."