Scientists said the situation is likely to get worse over the next few days as stagnant air from the Continent gets dragged over southern parts of the country.
Only Northern Ireland, northern England and Scotland are likely to escape from poor air quality this week, according to the National Environmental Technology Centre at Culham, Oxfordshire, which runs the Government's national air-monitoring network.
A particular problem is rising levels of ozone, a highly irritant gas produced by strong sunlight reacting with exhaust fumes. Ozone can trigger asthmatic attacks and damage crops.
Levels of nitrogen dioxide, a principal pollutant from car exhausts, were also high in central London, producing poor air quality at the network's monitoring site for 16 hours out of 24.
Jon Bower, head of air monitoring at the technology centre, said rural areas such as Devon, Oxford and East Anglia have registered ozone levels above 90 parts per billion, tipping air quality into the ''poor'' bracket.
Friends of the Earth said that over the past three days, ozone levels have exceeded international health guidelines at 26 of the Government's 32 monitoring sites. It called on the Government ''to reduce speed limits on motorways and restrain on-street parking in city centres''.
The pressure group accused the Government of putting out misleading reports on air quality. ''Its air quality bulletins have continued to describe air quality as 'good' even when international guidelines for ozone have been exceeded.''
The Met Office in Bracknell said the hot weather was likely to continue until the weekend. because of a large high pressure area extending from the Azores to Scandinavia.Britain, however, is not alone in experiencing a heatwave. Met Office scientists believe this reflects a situation often seen in the northern hemisphere in summer when four or five large areas of high pressubecome established simultaneously around the globe.Reuse content