A one-off expansion of the world's biggest marathon will see 100,000 participants take to the streets of the capital across two days, with organisers predicting record charity fundraising and a £400 million social and economic impact.

London Marathon Events has confirmed the TCS London Marathon will expand into a two‑day event for its 2027 edition.

Taking place on April 24-25 next year, this one‑off format change will see the London Marathon – already the most popular marathon on the planet – into a spectacular two-day celebration. The expanded event will further elevate the global impact of the event, which in 2025 was recognised as the No.1 marathon brand in the world.

The unique Double format will welcome 100,000 participants across the two days, all taking on the same iconic route, from Greenwich to Westminster, in what will be an unprecedented weekend of sport, charity and community celebration.

By extending the event across two days, the Double creates more opportunities than ever before to be part of the London Marathon. It will allow a record number of participants to take on a single marathon, helping to meet the extraordinary demand for places - highlighted by the fact that 1.33 million people entered the ballot for next year’s event.

The Double will also create the largest fundraising moment in UK sport. Already the world’s largest one-day annual fundraising event, with more than £90 million raised to date for charity by 2026 participants, this two-day marathon will give more charities more opportunities to raise vital funds.

The elite women, elite female para-athletes, championship and ‘good for age’ women will lead the mass event one day, with the elite men, elite male para-athletes, championship and ‘good for age’ men leading the mass event the other. All format details will be confirmed in the coming months.

Everyone who has already applied to take part next year will be entered into a ballot for both the Saturday and Sunday. Participants will not be allowed to take part in the in-person event on both days, though they will be able to complete the London Marathon one day and the virtual event, London Marathon MyWay, on the other.

Ballot results will be announced in early July, with places allocated through a random draw.

Organisers say the 2027 TCS London Marathon Double will be "a citywide celebration of sport, music, and community, with charity partners, creators and neighbourhood groups shaping activities across both days".

A major focus of the Double will be inspiring and supporting the next generation. Every school in London will receive two guaranteed entries for teachers or staff, recognising that the spark to get active often begins in the classroom.

All additional income generated by the Double will be distributed by the London Marathon Foundation to projects that inspire activity for children and young people across London and the UK.

To ensure local communities share in the occasion, extra guaranteed entries have also been allocated to every London borough along the route, enabling grassroots groups and charities to take part and benefit from the expanded event.

The Double will crown a full week of TCS London Marathon celebrations, beginning with an extended London Marathon Running Show and featuring London Marathon & Friday Night Lights 5km. The week will also include the TCS Mini London Marathon - the world’s largest one-day timed children’s event - which in 2027 will take place on Friday April 23 and will welcome more than 20,000 young participants.

Hugh Brasher, CEO of London Marathon Events said: “The 2027 TCS 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.

“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.”

Mayor of London, Sadiq Khan, added: “London is the sporting capital of the world and I am delighted that, for one year only in 2027, the world famous TCS London Marathon will expand into a two-day event.

“This unique, one-off double will allow a record 100,000 participants to take part across the two days - creating the largest fundraising moment in UK sporting history and supporting the work of so many incredible charities.

“The London Marathon showcases the very best of our capital and we will continue working closely with London Marathon Events and our partners to ensure appropriate measures are in place for the entire weekend as we build a better, more prosperous London for everyone.”

Brasher insists it will be a one-off, too. As he told the media in April, "It will be one and done."

The idea for the London Marathon to dominate two entire days in 2027 is hugely ambitious, but Brasher says he is inspired by a Robert Browning quote that his father, London Marathon founder Chris Brasher, was fond of: "A man's reach should exceed his grasp, or what's a heaven for?"

Despite the idea being talked about publicly earlier this year, London Marathon has worked hard to get its many and varied stakeholders on board with the idea. These range from the police and transport networks to the Mayor and London Boroughs.

Implementing the idea has been approached with caution, too, as Brasher explained earlier this year: "You have to bear in mind that the London Marathon has been going for 45 years and it’s got to a position of love, not only the sporting calendar but the calendar of London. You can lose that love very quickly and we have to be mindful of that. And that is why it is very much a one and done.”

It is also the perfect time for London to do this ambitious Double. One of the original founding pillars of the London Marathon was to show some joy and happiness to a somewhat troubled world. "And those words resonate more now in 2026 than they did in 1981," Brasher says.

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In an age dominated by social media and the advent of artificial intelligence, Brasher also feels people are likely to continue seeking real-life moments and experiences, such as running London, more and more.