London and the south east are set to avoid Saharan red dust from the continent which could cause problems for those with breathing difficulties, as experts warn of spikes in air pollution causing smog to build over Britain.

Asthma sufferers and older people are being advised to "reduce physical exertion" on what is expected to be the hottest day of the year, as the air becomes more dense with pollutants.

There are reports of a spike in people seeking medical help for hay fever in London.

Dr Sotiris Vardoulakis, head of the air pollution and climate change group at Public Health England's Centre for Radiation, Chemical and Environmental Hazards, said: "While most people will not be affected by short term peaks in air pollution, some individuals, particularly those with existing heart or lung conditions, may experience increased symptoms."

The Department for Environment, Food and Rural Affairs (Defra) has warned of spikes in air pollution across central, eastern and northern England.

Additionally, with summer seeming to come early for many this week, doctors have reported seeing hay-fever sufferers experiencing symptoms.

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, many people are already suffering from hay-fever symptoms.

Andrew Williams, nurse consultant at Homerton Hospital, east London, told the Daily Express: "People have already been coming in with symptoms triggered by the London plane trees.

"Now we are seeing high birch pollen levels, a lot of sufferers are presenting slightly earlier than usual as the problem gets worse."

The country could enjoy the hottest day of the year so far today as temperatures are expected to soar to 22C (71.6F), making parts of it warmer than Greece and Turkey.

The mercury is set to peak in parts of eastern England, making it warmer than the 6C (42.8F) forecast for Ankara and 17C (62.6F) in Athens.

However Brits should not pack away their umbrellas and raincoats just yet, as temperatures are expected to become more seasonal by tomorrow, with a band of rain expected overnight.

Met Office meteorologist Charles Powell said today's weather may be a shock to the system after a week of uncharacteristically warm temperatures.

He said: "On Friday we could see temperatures of up to 22C (71.6 F) across parts of eastern England, particularly to the north of London and into Cambridgeshire.

"But by Saturday morning temperatures will be lower after a band of rain overnight.

"But it's not that it's going to be particularly cold, it's just returning to nearer normal after a week of temperatures being above average, so it will feel slightly colder."

On Saturday temperatures of around 10C (50F) or 11C (52F) will be seen in the North and around 14C (57F) or 15C (59F) further south, providing some cooler air for spectators and competitors in the Grand National and the Boat Race.