This year's London Marathon will be the hottest on record, according to the Met Office.
Runners are being warned to take extra precautions as temperatures are expected to reach 23C (73.4F).
Temperatures of 22.2C (72F) were recorded in 1996 and 2007 - when one runner died after collapsing during the race and 73 were hospitalised.
The Met Office warned it will feel hotter on the course due to warmth "from the tarmac and other people".
Conditions may also be especially difficult for fancy-dress runners tackling the 26.2-mile event.
Almost 100 competitors are attempting to enter the Guinness World Records dressed in outfits such as a suit of armour, a Paddington Bear costume and ski boots.
Marathon director Hugh Brasher said entrants have been reminded of the advice to reduce their run goal times.
Mr Brasher added runners in fancy dress "should think carefully if that is appropriate in these conditions".
About 40,000 people are expected to take part in this year's race.
More water, ice and shower stations have been added along the route this year to help participants can cool down in the heat.
Met Office spokesman Grahame Madge said: "The advice to runners is to look at the forecast and prepare for the fact that this could be one of the warmer marathons."
Mr Madge said Sunday will start off bright with patches of clouds, but warned that later there could be some downpours due to "increasingly humid air".