Runners at this year’s London Marathon have been told to reduce their run time goals amid Met Office warnings that it could be the hottest ever.
In 1996 a highest temperature of 22.7C was set at the annual event, and it looks as though that record could be under threat.
Following a 29.1C record-breaking hottest April day for 70 years on Thursday, Met Office spokesman Grahame Madge said the mercury could rise up to 22C or 23C in the capital on Sunday.
Tens of thousands of runners are expected to take part in the marathon and race organisers have already announced they will add more water, ice and shower stations along the 26.2-mile route so participants can cool down in the heat.
This year's #LondonMarathon is set to be one of the warmest on record. Dry and sunny through the morning but with an increasing shower risk through the afternoon. Good luck to all the runners! pic.twitter.com/tWh5SgKgcx— Met Office (@metoffice) April 21, 2018
Marathon director Hugh Brasher also said entrants have been advised to reduce their run goal times.
He said: "We have emailed all our runners again this morning with detailed instructions so they can prepare for tomorrow.
"We have reminded them that they should adjust their goal for Sunday and plan to run at a slower pace and, if they were planning to run in fancy dress, they should think carefully if that is appropriate in these conditions.
"There is plenty of water available and runners should drink according to their thirst and use spare water to douse their head and neck."
Conditions may also be especially difficult for fancy-dress runners, including the almost 100 attempting Guinness World Records dressed in outfits such as a suit of armour, a Paddington Bear costume and ski boots.
Forecaster Mr Madge stressed that even if the mercury does not rise above the record, for runners it will feel "very, very warm" with “warmth coming up from the tarmac and other people”.
He said Sunday will start off bright, with potentially some higher cloud which "may reduce the extreme glare", but warned that later on there could be some downpours due to "increasingly humid air".
Set to increase in intensity and frequency throughout the event, he said the showers should bring respite for participants, but may not be so enjoyable for the spectators.
Additional reporting by Press Association