Runners who completed the Coventry Half Marathon on Sunday have been told by organisers there was an error measuring the length of the route.
Run For All, which operates events across the UK, admitted in a statement that the race "was longer than 13.1 miles".
The organisation offered an apology and explained how the mistake happened.
It said an initial investigation found a turning point was wrongly positioned by more than 250m, which meant the route was longer by over half a kilometre.
Run For All said: "We are disappointed that this has happened and apologise unreservedly that this has occurred and we take full responsibility for the error.
"Our initial investigation shows that the turning point was positioned incorrectly with it being 256m out, which has resulted in the route therefore being 512m long overall."
Runners have been told their results have been updated to provide a time equivalent to a 13.1-mile (21km) finish.
Participants have also been told they will be offered half-price entry to next year's run.
Organisers added they would be "running a full internal investigation... to ensure this doesn't take place again".
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The marathon, which began in the city centre before moving out to the northwest suburbs and looping back, coincided with a 10K run on the same day.
Run For All was created by the late amateur athlete Jane Tomlinson CBE, after her cancer diagnosis.
The company describes itself as a "fundraising vehicle for a huge number of local and national charities".
(c) Sky News 2026: Half marathon organisers apologise after Coventry route was set too long
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