Belfast City Marathon organisers have apologised after admitting that Sunday's course was 0.3 miles longer than it should have been.
Belfast Marathon chairman David Seaton blamed "human error", saying the lead car diverted from the route.
"Approximately 460 additional metres were added to the officially measured course of 26.2 miles," he said.
"This was due to human error, with the lead car diverting from the official route."
Earlier, John Glover, the event's course measurer, said runners had twice been "taken off the measured route".
"The route run was 469 metres in excess of the route measured and approved by the Association of International Marathons," said Mr Glover.
A distance of 469 metres equates to 0.293 of a mile.
Competitors times to be re-adjusted
In a statement, Mr Seaton said "protocols will be put in place to ensure this never happens again".
He added that race organisers were in the "process of adjusting runners' times to reflect the correct distance".
Following Sunday's race, a number of questions were raised on social media about the new course's length.
In 2013, 2014 and 2015, the Greater Manchester Marathon course was 380m too short, as a result of a measuring error. UK Athletics subsequently declared the times of those races invalid.
Kenyans Kositany and Jepchirchir secure wins
Kenyans Joel Kositany and Caroline Jepchirchir took victory in the first Sunday running of the event.
Kositany secured his fourth Belfast men's triumph as he crossed the line in two hours 18 minutes 40 seconds.
Jepchirchir repeated her 2018 win as she set the fastest ever women's time in Belfast, clocking 2:36:38.
This 38th staging of the event took place on a new course which organisers hoped would ensure faster times.
However, the discrepancy with the course distance is now likely to be the main talking point following the race.
Event chairman Seaton admitted that the mistake will upset a number of competitors.
He said: "I can understand if you have been aiming for a sub three-hour marathon time and because of the mistake you have ended up being just outside three hours on the clock, that you are going to be annoyed.
"It's a hiccup that we obviously could have done without. But I don't think it should overshadow what was a very successful day with the numbers up significantly because of the new Sunday date.
"People have been coming up to us congratulating us on the day and saying it was a great event with the spectator number also well up on previous years."