Warning comes as air pollution is expected to reach eight or nine on a 10-point scale in much of southern UK

Millions of vulnerable people in southern England and Wales were advised to stay indoors as unusually high levels of air pollution smothered London and other cities, just weeks after heavy pollution led to restrictions on car use and the offer of free public transport in Brussels and Paris.

Prof Frank Kelly of King's College London said tiny particles of pollution could cause problems for some people – such as those with asthma – and contribute to longer-term problems for others.

"Periods of very high pollution in London arise usually because the city's normal pollution emissions are increased further by polluted air arriving on light winds from neighbouring areas such as northern Europe," he said. "However, whether home-produced or arriving from the continent, the tiny particles we take into our bodies with each breath cause immediate problems for some individuals, such as those with asthma, and contribute to longer-term problems for most of us in the form of heart disease and stroke."

Leanne Stewart, from Eltham in south-east London, described feeling breathless after a routine half-mile walk to her son's thisschool yesterday morning.

"I've been doing the usual school run about half a mile from my house, which is usually quite an easy walk, but I'm still breathless now," she said. "I could feel my chest getting tighter and tighter and my son, who's eight, had to stop and have his inhaler.

"I went light-headed and had to get a bus back. It's only half a mile and I usually do it twice a day, no problem. I've never had that problem before. My son felt like the air wasn't getting into his lungs so I'm worried about him today, but I've just texted him and he said he's fine."

Experts said this week's air pollution was not unusual for this time of year, but the difference was that the public was being informed because of the Met Office's new forecasting system.

"Two weeks ago, when the air pollution map of the south-east turned red, no one was informed," said Dr Ian Mudway, lecturer in respiratory toxicology at King's College London. "The BBC ran stories about the pollution in Paris and Milan, but no one thought it worthwhile to inform the British public that they were being exposed to dangerous levels of fine particulates [tiny pieces of liquid or solid matter].

Daily air quality index for 2 April 2014. Source: Defra

"At-risk groups were not informed, which to my mind is unforgivable. My suspicion was that there was an almost 'what the public don't know they can't complain about' attitude. Thanks to the new forecast, air pollution is going to ratchet very rapidly up the public consciousness and it will be interesting to see how government, both local and national, responds."

On 1 April, the Department for Environment, Food and Rural Affairs (Defra) handed over responsibility for reporting air pollution to the Met Office, a move welcomed by Alan Andrews at the environmental law firm ClientEarth.

"I think it has to be a good thing, because the Met Office are good at communicating to the public and it depoliticises it; it takes away the issue of a minister worrying about looking bad because of putting out a pollution warning."

Forecasters warned that one of the year's worst smogs would worsen as Wednesday progressed, with air pollution reaching the maximum "very high" level in many parts of southern Britain.

The unusually high levels of air pollution are a result of inland pollution and powerful dust storms swept in from the Sahara by strong gales.

"We usually see this happen several times a year when big dust storms in the Sahara coincide with southerly winds to bring that dust here," said Paul Hutcheon at the Met Office. "More dust rain is possible during showers expected later this week."

Defra said air-pollution levels on Wednesday were expected to reach eight or nine on a 10-point scale in much of southern England, the Midlands and southern Wales.

The south coast is forecast to see pollution hit the maximum level 10.

Adults and children with lung problems, heart defects and older people are being urged to avoid any strenuous activity and asthmatics have been told to use their inhaler more often.

Advice on the Defra website for the general population facing "very high" pollution levels states: "Reduce physical exertion, particularly outdoors, especially if you experience symptoms such as a cough or sore throat."

Defra forecasters issued the fresh warning after air pollution levels reached 10 in north-west Norfolk on Tuesday.

"For England and Wales, moderate to high air-pollution levels are forecast for central and south-east England, to the south and east of a line from around the Wash to Cheshire to east Devon, with local very high levels expected in parts of East Anglia and the East Midlands," said a Defra forecaster.

Conditions are expected to worsen on Wednesday evening and into Thursday as the smog extends north.

High levels of pollution are forecast to stretch across East Anglia, the Midlands, Lincolnshire, Wirral and north and east Wales, then further north over much of England's north-west coast, south-west Scotland and the north-east of Northern Ireland.

Cleaner air from the Atlantic should bring some respite for south-west Wales and England's southernmost counties on Thursday, forecasters said, although many other parts of England and Wales will experience higher levels of smog than usual.

Keith Taylor, the Green party's MEP for south-east England, said the high level of pollution showed the need for stronger government action.

"Episodes like this often have a number of contributing factors," he said. "What seems clear is that towns and cities across south-east England need to reduce the level of air pollution in their streets by cutting the amount of traffic. But on top of that it's clear that our government must play a role in pushing for stronger air pollution laws from the European Union, rather than trying to water down the rules we already have."

The UK faces fines of up to £300m a year and embarrassing court appearances after the European commission launched legal proceedings against it for failing to reduce "excessive" levels of nitrogen dioxide air pollution from traffic, despite 15 years of warnings and several extensions and postponements granted to the government.

According to the World Health Organisation, air pollution has become the world's single biggest environmental health risk, linked to about 7 million deaths a year – or nearly one in eight deaths in 2012.