The Met Office has said that the level of tree pollen – specifically birch and ash – in the capital and across the south east is expected remain high throughout the week.
The mercury is set to peak in London at 25C on Thursday, when the pollen count will be at its highest, before warm and sunny conditions remain into the weekend.
The pollen count in the north east of the country – where wind and rain is forecast – will be lower.
Forecaster Sophie Yeomans told the Standard: “For today there will be high pollen levels in some parts of the UK for south and eastern areas for tree pollen.
“In the south east where we will have the best of the sunny weather the current allergens are birch and ash.
“The rest of the week could stay fairly high in the south, with the north west looking lower as there is some rain around there, but down in the south it is going to be high this week.”
The symptoms for hay fever include a runny nose, itchy eyes, coughing and sneezing.
The best sunsets in the world - in pictures
NHS advice for sufferers includes putting Vaseline around nostrils to trap pollen, wearing wraparound sunglasses to stop it getting in your eyes and staying indoors when possible.
Temperatures are expected to remain above 20C in the south east from Wednesday until next week, meaning Sunday’s London Marathon could be the hottest on record.
The mercury could reach as high as 21C on Sunday, with the highest recorded temperature on marathon day 21.7C on April 22, 2007.
The south east of the country is expected to experience the best of the weather, while the north west will see wind and rain before brighter weather arrives from Wednesday.
