He said: “Grass pollen season starts around the second half of May and this could take us to the end of July.
“Grass pollen affects many more people with around 95 per cent of hay-fever sufferers allergic to grass compared to 25 per cent who are allergic to tree pollen.
“Right now, people who are allergic to tree pollen are having a bad time especially with exam season about to get underway.
“This could get worse as we go through the second half of May and into the start of summer.”
Pollen counts will rise across eastern England ahead of the weekend with moderate levels expected elsewhere, according to the Met Office’s forecast.
Temperatures will fall widely into the mid-teens over the coming days while the southeast holds on to the best of the weather.
Conditions could turn more unsettled over the weekend bringing the risk of thunderstorms into next week, government forecasters warned.
Met Office spokeswoman Nicola Maxey said: “There is a chance that this weekend will bring the risk of thunder and hail.
“A band of rain will pass eastwards on Saturday bringing the heaviest showers to the northeast while across the south we expect sunshine and showers, it is here where there is the greatest risk of thundery outbreaks.
“It is a similar picture on Sunday with another slow-moving band of rain and the risk of thunder in southern and western parts.
“Again on Monday there is a risk of thunder in central and southern regions.
“Temperatures will be highest in the southeast, we could see 20C (68F) in London, elsewhere we expect highs of between 15C (59F) and 17C (62.6F).”
Allergy UK urged people suffering from asthma and pollen allergies to keep medications to hand as the weather changes.
Allergy UK Nurse Advisor Holly Shaw said: “Prolonged periods of warm, dry weather can lead to higher levels of pollen in the atmosphere.
“When the weather conditions change, for example when thunder storms are predicted, this can be a risk factor for those with hay fever and allergic asthma which can be triggered by small pollen particles that are breathed in through the nose and taken deep into the lungs where they cause inflammation which can trigger asthma symptoms.
“It is important for those with both hay fever and asthma that their hay fever is well controlled, if not this can effect asthma control and may lead to an asthma attack.
“It is important to ensure that when these weather changes are forecast those living with hay fever and asthma carry their allergy medication is available at all times, recognise their allergy symptoms early.”