Temperatures to peak tomorrow - but the sun isn’t here to stay

And the forecast has been accompanied by a warning of soaring levels of air pollution.

Met Office meteorologist Craig Snell said today would see the mild conditions experienced in most areas over recent days continue, with temperatures at around 19C (66F) or 20C (68F).

He added: “On Friday temperatures will be a little bit higher with highs possibly reaching 21 and if you do see 21 that will mean it is the warmest day of the year so far.”

On current forecasts it could mean Britain is warmer than holiday destinations such as Ibiza and Crete as well as Barcelona and Rome.

But on Friday night fresher air will arrive across the country and on Saturday most parts will be cooler, with temperatures around 10C (50F) or 11C (52F) in the north and around 14C (57F) or 15C (59F) further south.

Mr Snell said this was likely to be followed by “unsettled” conditions across the country on Sunday, with outbreaks of rain possible.

He said: “It is pretty normal for April to see some warmer weather followed by some colder weather.”

While the pollen levels are moderate for much of the south of England and low for the rest of the UK according to the Met Office, doctors have reported seeing hay-fever sufferers experiencing symptoms.

Very high levels of air pollution are expected in some areas of south east England, as still, warm conditions create a build-up of local pollutants from sources such as traffic fumes, combined with pollution from the continent and Saharan dust from the south.

Much of the South East and eastern England will see high levels of pollution, although the problem is expected to be short-lived, with Atlantic winds dispersing the polluted air in the early hours of Saturday, the Environment Department (Defra) said.

In areas experiencing very high levels of air pollution, adults and children with lung problems, adults with heart problems and older people are advised to avoid strenuous activity.

People are also advised to reduce physical exertion, particularly outside, and asthma sufferers may need to use their reliever inhaler more often.

Where pollution levels are high, adults with heart or lung problems and children with lung problems should reduce strenuous physical activity, older people should reduce physical exertion and anyone suffering from sore eyes, a cough or sore throat should cut down on activity, particularly outdoors.

A Defra spokeswoman said: “Locally generated air pollution, combined with pollution from the continent and Saharan dust, could cause high or very high levels on Friday.

“This is expected to clear on Saturday and pollution levels will return to low throughout the morning.”