With the touch of a button the Queen will launch the London Marathon, sending a tide of more than 40,000 runners through the capital.
The 38th edition of the world-famous race may well be the hottest on record, with sweltering highs of 23C possible throughout Sunday.
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.
Among this year's runners are firefighters who tackled the Grenfell Tower blaze, a police officer stabbed in the London Bridge terror attack and members of the Stephen Lawrence Charitable Trust.
Sunday marks 25 years since the murder of the teenager, who was a keen runner.
Meanwhile almost 100 runners will attempt Guinness World Records - dressed variously in suits of armour, as mythical creatures, and wearing stilts and ski boots
Hundreds of police officers will be on duty to keep around 800,000 spectators and runners safe.
Competitors will start from Blackheath, south east London, running a snaking route along both sides of the Thames, finishing on The Mall near Buckingham Palace.
Last year, the event raised £61.5 million for charity, a world record for an annual one-day fundraising event, making the total raised since 1981 around £890 million, organisers said.
A record 386,050 people applied for this year's race - almost a third more than last year and the highest number for any marathon in the world.
Races start from 8.55am.