The current brilliant hot spell is set to peak is forecast to be hotter than Spain and Italy.
Hay fever sufferers could be affected by high pollen counts, he added, saying: “For anyone who suffers from tree pollen they will probably be feeling the effects, but that’s only about 20% of hayfever sufferers. Grass pollen season comes later in summer.
Weather experts expect temperatures to beat Wednesday’s highs of 25C in both Gravesend in Kent and St James’s Park in London – the warmest day of the year so far.
In the south-east thermometers are expected to hit 26C, 27C or 28C, beating the UK’s warmest April day of 29.4C recorded in 1949, the 2001 April high of 27.1C.
The London Marathon on Sunday is being urged to reconsider after warnings it could be the hottest race on record. Competitors have been told to expect hot and humid conditions of between 21C and 23C.
The event director, Hugh Brasher, said: “The previous highest temperature on Raceday was 22.2C in 1996 and 2007. We will continue to monitor the forecast closely as we all know that weather forecasts can change.”
He added: “Our contingency plans for the forecast hot weather on Sunday include increasing the supply of water to all drinks stations to enable runners to douse themselves with water, increasing the number of showers on the route and increasing the supply of ice to first aid stations on the route.
“Most importantly, runners should adjust their goal for Sunday and plan to run at a slower pace. If they were planning to run in fancy dress, they should think carefully if that is appropriate in the forecast conditions.”
Only last month, the UK was struggling with snow and ice from the “beasts from the east”, which prolonged the winter well into March.
Now, summer appears to be on the horizon. Northern Ireland likely to push 19C in some parts and a peak of 20C possible in Aberdeenshire in Scotland, the forecaster said. The average maximum temperature for April, which is 11.4C.
The summery spell is caused by warm air from the Azores being dragged towards the UK due to an area of low pressure over the Atlantic and high pressure over western Europe.
Meteorologist Alex Burkill said: “There’s a fairly good chance of 28C, there’s about a 60% chance. Quite widely we are going to see low 20s, and for many, it will be a little warmer than Wednesday.”
Some hayfever sufferers could be affected by high pollen counts, he added, saying: “For anyone who suffers from tree pollen they will probably be feeling the effects, but that’s only about 20% of hayfever sufferers. Grass pollen season comes later in summer.”