Thousands of runners who took part in the Marathon of the North races have been told the course they followed was 264m short.
Competitors in the full and half marathons took a wrong turn in the Sunderland events because marshals were standing in the wrong positions.
Only the winner of the full marathon completed the full distance.
Organiser DS Media & Events apologised and has offered affected runners a 25% discount on next year's entrance fee.
In a statement, it said: "Only the lead athlete followed the correct route through the Sheepfold area near the Stadium of Light.
"Unfortunately, the second and third placed runners were not within line of sight of the leader and lead bikes and vehicle.
"This resulted in all those who followed taking an incorrect route through this section."
Jake Harrison of Leicester Tri Club completed the full course and won the marathon.
About 5,000 runners took part in the races, which started and finished at the Stadium of Light.