Runners are being warned that temperatures on Sunday could exceed the warmest London Marathon on record, with a chance of the mercury rising to 23C or higher at some points along the route.
Thundery showers overnight on Saturday means that temperatures are likely to be high after a humid start, Met Office forecasters have warned.
The warmest London Marathon on record was in 1996 when it reached 22.7C in St James’s Park.
Runner’s World has warned competitors tomorrow to be aware of the impact the warm weather can have on them.
Every 2-3C rise in temperature above 15C can slow a runner’s pace by as much as 20 to 30 seconds per mile.
The Met Office added: “After a dull and cold start to spring it is important to remember that the sun is as strong in April as it is in late August.”
This week organisers announced they will add more water, ice and shower stations along the 26.2-mile route.
Elsewhere temperatures will have started to decline slightly with high teens in large parts of England and Wales and mid-teens in Northern Ireland and Scotland.
“Many people will be happy to hear that the warm conditions are forecast to stay around for the weekend, although there will be some heavy thundery showers to watch out for at times,” Met Office chief meteorologist Will Lang said.
“For those running the marathon, make sure you stay up-to-date with the forecast as there will be high temperatures and humidity to contend with. Stay hydrated and listen to advice from the event organisers.”