Organisers of the Edmonton Marathon have admitted their course this year was more than half a kilometre too long.

The annual event in Alberta, Canada is often used as a qualifier for the prestigious Boston Marathon.

But a number of frustrated runners voiced concerns online after the 16 August race, noting that the distance on their GPS running watches was over 26.22 miles.

Organisers initially dismissed complaints, but in a statement on Wednesday they acknowledged an extra 674.7 metres (0.41 miles) was mistakenly added to the course.

Race director Tom Keogh said: “We do not take this lightly. We hosted the nation’s best athletes and their finishing times did not truly reflect their amazing performances.”

Keogh added that organisers “did not deliver the standard participants deserved”.

Construction work around the Wellington Bridge was cited by organisers as the main reason for the error, which forced a slight detour to the course’s usual route.

The extra distance means participants may have missed out on qualifying for the Boston Marathon, as well as personal best times.

Qualifying times for Boston are based on a participant’s official chip time at qualifying events – not the time recorded on their watch.

Organisers say they are working with Athletics Canada and other partners to determine how this year’s results can be recognised.

BBC Sport has contacted the Edmonton Marathon for comment.