A charity is investigating after 2,000 runners who thought they had completed a 5km (3.1 mile) run found out it was 1.2km (0.7 miles) too short.
A section of tape marking out the route at Victoria Park, Leicester, was removed without permission during the Race for Life event on 7 July.
The runners, who were fundraising for Cancer Research, were sent down a route that fell short of the full race.
The charity has apologised to all who took part.
Amy Hall, Cancer Research UK’s Leicester event manager, said: “We spend a lot of time and effort planning each of our Race for Life events to make sure they are professional, safe and well-communicated.
“After the 5km event had started we discovered a section of marker tape had gone missing.”
The route was supposed to be the same as the earlier 10km (6.2 mile) event, where 1,000 runners did the course twice.
The charity said it hopes missing out on a section of course did not detract runners from the “incredible atmosphere” that came from the event.