Hay fever cases have more than doubled since this time last year, with a fivefold rise in cases in a fortnight.
Experts say a rainy spring with little frost, followed by early sunshine, has created optimum conditions for repeated high pollen levels - meaning Britain could face one of its longest hay fever seasons for years.
Charities said current forecasts of changeable weather, mixing sunshine and showers, were the "worst scenario" for Britain's 18 million hayfevers, and likely to mean that the season lasts for longer.
Pollen counts are currently very high in most parts of the UK, apart from Scotland, and forecast to remain medium to high for the rest of the week.
New figures from the Royal College of General Practitioners (RCGP) reveal a huge spike in the number of patients seeing their GP because of symptoms of hay fever.
Almost 12,000 patients saw family doctors about the condition in the second week of June – more than twice as many as during the same period last year.
Doctors said the number is just a small proportion of the numbers struggling to cope with sneezing and itching, since most people would visit a chemist rather than a GP to seek help for the condition.
Professor Jean Emberlin, scientific director of Allergy UK, said the rise in numbers seeking help from their GP suggested increasing numbers of people suffering from hay fever for the first time, or after years when symptoms had been more manageable.
She said: "I think we are looking at a severe year. Last year was the worst we had seen since 2004 in terms of pollen count, and the worst scenario now is if we keep getting short periods of rain, followed by sunshine and a breeze. That kind of off/on weather starts it all off again."
Experts said Britain was likely to be in the peak of the season now, but that the weather would determine how long it continues.
The figures show that between 9 and 15 June, 11,873 people in England and Wales visited their GP because of hay fever, compared with 5,560 people this time last year – a rise of 114 per cent.
Grass pollen is the most common cause, which tends to affect people from around May to July. Tree pollens tend to affect people from March to May and weed pollens from early spring to early autumn.
Symptoms include sneezing, a runny nose and itchy eyes, caused by the immune system reacting to the pollen. Cells on the lining of the nose and eyes release histamine and other chemicals, causing inflammation in the nose and eyes. Sometimes the sinuses and throat can also be affected.
Maureen Baker, chairman of the RCGP said: “Each year, seasonal hay fever causes untold misery to thousands of people across the country, especially when we all want to enjoy the warm weather.
“Hay fever is awful but the discomfort should only be temporary and there should be no long-term effects. Whilst in some cases it may be necessary to see a doctor, especially if the symptoms persist, there are many antihistamine medications that can be bought over the counter at your pharmacist that should provide effective relief.
"Patients that suffer from hay fever can also take simple steps to help minimise their exposure to pollen, such as wearing a hat with a wide brim and sunglasses, and to applying Vaseline to nostrils to help trap pollen particles.”