The London Marathon will be held across two days in a special one-off edition of the event in 2027.
Organisers
confirmed earlier this year that they were exploring ways to enable more people to take part, after a record 1.33 million people entered the ballot for next year's event.
The two events - on Saturday, 24 and Sunday, 25 April - will allow 100,000 people to take part in the London Marathon in 2027, effectively doubling the chances of success for those who entered the ballot.
The move is also expected to raise more than £150m for charities and provide a £400m boost to the UK economy, according to Hugh Brasher, chief executive of London Marathon Events (LME).
Everyone who applied to take part next year will be entered into a ballot for both the Saturday and Sunday, with the ballot results announced in early July.
"The 2027 London Marathon double is our most ambitious evolution to date - a once-in-a-generation one-time-only reimagining of what a marathon and city-wide celebration of activity can be," said Brasher.
"By expanding to 100,000 runners across two days, we're opening the door for more people, more charities and more communities to take part in the world's greatest marathon. We believe that more than £150m can be raised for good causes and the UK economy will have a £400m social and economic benefit.
"We are grateful for the vision and support of the Mayor of London, the councils on the route, the emergency services, and all the stakeholders involved who have given their support to enable this world first event to be held for London and the UK."
Sadiq Khan, the Mayor of London, said: "London is the sporting capital of the world and I am delighted that, for one year only in 2027, the world famous London Marathon will expand into a two-day event."
A total of 1,338,544 people applied for the 2027 London Marathon, breaking the record of 1,133,813 applicants for the 2026 event.
The London Marathon had a record 59,830 finishers in 2026 and was the biggest annual one-day fundraising event in the world, with more than £90m raised to date for charity by 2026 participants.
History was also made on the course at this year's event, as Kenya's Sabastian Sawe became the first athlete to run
a sub-two-hour marathon in a competitive race.
Brasher has remained adamant that the expanded two-day event will be for one year only, following what he described as an "astonishing total of applicants" hoping to take part in 2027.
He told BBC Sport the two-day event has been nine years in the making, but after the initial idea was formed in 2017, with the intention to hold it in 2020, they failed to gain permission.
The plan is for the elite women, elite female wheelchair athletes, championship and 'good for age' women to lead the mass event one day, with the elite men, elite male wheelchair athletes, championship and 'good for age' men leading the mass event on the other.
However, London Marathon organisers said all format details will be confirmed in the coming months.
All 100,000 participants across the two days will run on the normal London Marathon route from Greenwich to Westminster.
Brasher clarified to the BBC that a ballot draw will first be held for the Sunday - the day on which the marathon is normally held - before a second draw takes place for the Saturday.
The Mini London Marathon will be held on the Friday, with more than 20,000 young participants.
Organisers say all additional income generated by the two-day format will be distributed by the London Marathon Foundation to projects that inspire activity for children and young people across London and the UK.
www.bbc.co.uk
