As the Queen fired the starting gun on this year’s London Marathon, forecasters were predicting it could be the hottest on record and will feel very warm for the runners on the capital’s streets.
In 1996 a highest temperature of 22.7C was set at the annual event, and it looks as though the record which was set 22 years ago could be under threat.
Runners have been advised to drop their goal-times and organisers have added more ice, water and run-through shower stations along the 26.2-mile course.
Following a 29.1C record-breaking hottest April day for 70 years on Thursday, Met Office spokesman Grahame Madge said the mercury could rise to 22C or 23C at weather stations in the capital on Sunday.
“But a big caveat here is obviously a weather station temperature record will feel a bit different to what it might feel like trackside where you have the warmth coming up from the tarmac and other people,” he said.
“Trackside temperatures will probably feel even warmer than that, so 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 it is possible there could be a new warmest temperature, but stressed that even if the mercury does not rise above the record, for runners it will feel “very, very warm”.
Tens of thousands of runners are expected to take part in the arduous feat as they pound the course around much of London, cheered on by thousands of supporters.
Race organisers have announced they will add more water, ice and shower stations along the 26.2-mile route so participants can cool down in the heat.
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.
Marathon director Hugh Brasher said entrants have been reminded of the advice 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.”
Mr Madge 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.
Elsewhere across the weekend, a yellow warning for rain has been issued by the Met Office covering northern England and North Wales.
Heavy downpours and thunderstorms are expected to develop on Saturday afternoon and overnight into Sunday, with the warning in place from 4pm until 3am.
The Met Office warns there could be some flooding to homes and businesses, potentially loss of power from lightning strikes and poor driving conditions caused by spray from the rain water.
Along with hail and gusty winds in some places, between 15 and 20mm of rain may fall in a short space of time, with up to 30mm in some areas, the Met Office said.
From next week there will be cooler temperatures and a “more unsettled outlook” from Monday, Mr Madge said.