Organisers have announced they will add more water, ice and shower stations along the 26.2-mile route given that highs of 23C are possible, beating the 1996 record of 22.7C.
Conditions may be especially difficult for fancy-dress runners, including the almost 100 attempting Guinness World Records dressed in outfits like a suit of armour, a Paddington Bear costume and ski boots.
Professor Sanjay Sharma, Medical Director of the London Marathon said: “If you were planning to run in fancy dress, please think carefully whether that is still appropriate in the conditions.”
Regular runners are being advised to consider dropping their goal times and to run more slowly.
Thursday was the hottest April day for nearly 70 years, with the mercury soaring to 29.1C in St James’ Park in London.
Friday’s highs are likely to be around 28C in the south east, the Met Office said, with most parts enjoying warmer conditions than normal for the time of year.
“We will see highs of 26C, 27C and maybe 28C in the London area. Low 20s are likely for much of England and Wales, while Northern Ireland and eastern Scotland will see temperatures in the high teens,” said meteorologist Alex Burkill.
All four home nations enjoyed their hottest day of the year so far, with everywhere seeing a high of at least 20C on Thursday.
The hot weather, which is a result of warm air moving up from the Azores in the south, is set to last into the weekend.