Residents living in 24 cities across north-east China have been told not to leave their homes after thick grey fog engulfed the region.

The build up of the most severe episode of poisonous smog in a year has prompted a red alert warning, triggering the closure of schools and restrictions on road traffic.

Some 460 million people have been affected by pollution levels that environmental group Greenpeace claimed were six times higher than the World Health Organisation's guideline levels.

China declared a “war on pollution” in 2014 amid concern its heavy industrial past was tarnishing its global reputation and holding back its future development

But it has struggled to reverse the damage done by decades of breakneck economic growth, much of it based on the coal-burning power sector.

The smog – which has shrouded the area for four days– has grounded more than 300 flights from the country.

The chief executive of the world’s biggest oil exporter was also caught up in the disruption, with his plane prevented from landing in Beijing on Tuesday due to the heavy smog.

Amin Nasser, CEO of the Saudi Arabian oil producer Aramco, was due to attend an event on Saudi culture at Beijing's National Museum.

China’s environment ministry has warned that firms were flouting emergency restrictions.

Some power plants and chemical producers had not scaled back operations in line with new regulations, according to China Environmental News, the official publication of the Ministry of Environmental Protection.

The region affected includes several major cities in the industrial province of Hebei, which surrounds Beijing.

Red alerts are issued when an air quality index (AQI) is forecast to exceed 200 for more than four days in succession, 300 for more than two days or 500 for at least 24 hours.

In Handan, a major steel producer, the 24-hour average AQI at one monitoring station reached a record 780, Greenpeace said.

“The scale of the red alert measures show that the Chinese government is taking air pollution seriously,” said Dong Liansai, a climate and energy campaigner with the group.

“However, the ongoing 'airpocalypse' is further evidence that China must implement far stricter limitations on coal consumption and accelerate the restructuring of the economy away from the heavily polluting sectors,” Mr Dong added.

Pollution alerts have become common in China's northern industrial heartland, especially during winter when energy demand – much of it met by coal – skyrockets.

Official data from the Hebei provincial capital of Shijiazhuang showed average readings of small breathable particles known as PM 2.5, a major components of China's AQI, had risen to 665 micrograms per cubic metre.

The World Health Organisation recommends concentrations of just 10 micrograms.

The government said average PM 2.5 concentrations in the first three quarters of 2016 dropped 8.5 per cent compared with the same period last year, but officials have expressed concern that heavy winter pollution has overshadowed any progress.

China has repeatedly blamed “unfavourable weather” for much of the pollution this month, with high humidity and low wind speeds preventing the dispersal of emissions from coal-fired winter heating plants.

Several lawyers are suing the cities of Beijing and Tianjin and Hebei province for what they say has been their failure to fulfil responsibilities in battling the smog.

London smog: Then and now Show all 18 1 /18 London smog: Then and now London smog: Then and now London A London policeman wearing a mask for protection against the thick fog which hit most of the country and turned to smog in the city London smog: Then and now London A cyclist wears a mask with an air filter as he cycles through Hyde Park London smog: Then and now pollution-UK-2.jpg Heavy smog in Piccadilly Circus, London, 1952 London smog: Then and now London The Shard and St Paul's Cathedral from Hampstead Heath in London London smog: Then and now London A couple of office workers wearing masks to protect them from the dangerous smog in London, 1953 London smog: Then and now London A cyclist wears an anti air pollution mask as she cycles along The Mall London smog: Then and now London A London bus makes its way along Fleet Street in heavy smog,1952 London smog: Then and now London A view of London skyline covered in smog.The environment department confirmed that the air pollution level could reach the top rung on its 10-point scale London smog: Then and now London Morning traffic at Blackfriars, London almost at a standstill because of the blanket smog,1952 London smog: Then and now London Dust settled on a vehicle in South Kensington, which has blown up from the Sahara desert London smog: Then and now London A tugboat on the Thames near Tower Bridge in heavy smog, 1952 London smog: Then and now London A view of the Tower Bridge as the country continues to experience 'very high' levels of pollution London smog: Then and now London A couple wearing masks to protect them from the smog in Blackfriars,1954 London smog: Then and now London Tourists wear face masks London smog: Then and now London A woman wears mask in London, 1953 London smog: Then and now London A woman wearing an anti-pollution mask rides a bicycle at Hyde Park Corner London smog: Then and now London Smog masks have become all the rage in London due to the life threatening levels of air pollution with result in severe smog or 'pea soupers', 1953 London smog: Then and now London A couple stands on the viewing platform of a skyscraper

In recent days, Beijing has imposed an “odd-even” licence plate restriction in a bid to remove half the vehicles from the roads, but there have been at least 85,000 cases of violations.

The capital has also suspended outdoor operations at more than 3,000 construction sites.

According to forecasts, the pollution will gradually abate with the expected arrival of northerly winds on Thursday.