Air pollution warnings have been issued with temperatures forecast to climb towards 27C (80F) in south-east England and the Midlands this weekend.

Some parts of Britain will be hotter than areas of the Mediterranean, but the warm weather will be accompanied by moderate levels of air pollution, which can cause breathing difficulties in vulnerable people. Areas of south-west England and western Scotland could be at higher risk by Sunday. The highest levels are expected in Northern Ireland.

Thursday was the warmest day of the year so far, with temperatures topping 20C, bringing Britons out to enjoy the sunshine in parks, streets and on beaches. Sunshine activates photochemicals in polluted air, however, creating problems with pollutants such as ozone, which can cause shortness of breath in susceptible people.

The Department of Environment, Food and Rural Affairs (Defra) said those with lung or heart problems who experience symptoms should consider avoiding strenuous activity, particularly outdoors.

A department spokesman said: “Strong south-easterly winds blowing in air from the continent could lead to moderate levels of pollution … over the weekend in parts of England and Wales. Levels are expected to fall after the weekend. This does tend to happen during the change in the temperature around springtime.”

Pollution levels were classed as moderate over much of south-east England on Thursday, as winds brought air from continental Europe laced with industrial and agricultural pollutants. These chemicals then combined with locally produced pollutants, such as particulates and nitrogen oxides from diesel vehicle engines, to produce a toxic mix.

Diesel cars produce particulates – tiny pieces of unburned fuel that can lodge in the lungs and cause breathing problems – and nitrogen dioxide, another pollutant gas that affects breathing.

Gary Fuller, of King’s College London, said: “As spring is moving towards summer the sun is getting stronger, and able to drive chemical reactions between pollutants that cause ozone to be formed, along with the particles [of unburned fuel].”

Simon Birkett, director of the campaigning group Clean Air in London, said there should be clearer public warnings about levels of pollutants: “This is the fourth air pollution episode this year, and the first summer ozone episode,” he said. “It is expected to reach moderate or high levels, and last through the weekend until the wind speed increases on Sunday evening or Monday. It may come and go into the following weekend.

“People may experience tightness in their chest or shortness of breath, and would be sensible to carry their medication if they are asthmatic. Organisers of marathon, half-marathon and other long-distance events this weekend should warn participants.”

Defra tweeted that pollution levels were low across the country on Thursday morning, prompting campaigners to accuse the agency of playing down the threat of air pollution levels, which rose during the day.

Birkett said: “Defra hasn’t published an annual media release warning of the first summer smog episode – like this one – since 2011. This is a national disgrace and explains why so many people are confused by Met Office, government and other forecasts that omit air pollution warnings or show pollution as low relative to alert levels, not according to [international standards].”