A half marathon involving 15,000 runners has turned out to be too short - by 193m (211 yards).
Barnardo's, organisers of Sunday's Cardiff half marathon, which attracted elite athletes, club runners and charity fun-runners, have apologised.
They said a last-minute "urgent decision" was taken to change the route on safety grounds due to an obstruction on the course.
Runner Mick McGeoch, 55, said: "When I finished I knew it was short."
A race spokesman said: "The safety of all runners and spectators was our primary concern when making such decisions and the change in the course had to be undertaken when an obstruction was identified by race organisers.
"The obstruction resulted in the urgent decision to change the route to ensure there were no risks to runners."
The organisers said an investigation into the route discovered that the length of the 13.1 mile course was reduced by 0.12 miles.
Club runners had raised concerns after Sunday's race, which was held through Cardiff city centre to Cardiff Bay.
During the race Mr McGeoch, a founder member of the Cardiff-based Les Croupiers running club, said he thought the mile markers for the race were out but when the race ended he knew for sure the distance was short.
He told BBC Radio Wales: "I've run so many of these races over the years I can predict to a very high degree of accuracy what time I'm likely to run.
"Last year I was an official pacemaker for the race to run one hour and 20 minutes and I run that exactly. This year I was about one minute and 10 seconds faster."
Asked if he was bothered, Mr McGeoch, added: "I certainly do mind. There is a bond between race organisers and the runners. There's a faith if you like but when things don't go well you lose faith in their organisational skills.
"We're also very, very aware that an awful lot of time and effort and money went into it and frankly, the race and the runners and everybody actually deserve better."
Club runners were sent an e-mail from the organisers saying they were investigating on Monday .
This was the first year the road race was granted championship status by Welsh Athletics, the governing body for athletics in Wales.
This meant that the first three Welsh men and women to cross the line who are registered with Welsh Athletics qualified for Welsh Championship medals.