Newport Half Marathon route was 636 metres short
Newport's half marathon course was 636 metres short of the required 13.1 miles, it has been confirmed.
Phil Cook, of the Association of UK Course Measurers, re-measured it after runners questioned the distance.
Mr Cook said he could not pinpoint a specific location where an error was made by organisers.
He said: "This obviously doesn't affect people who ran for charity. But if it was for an official time, unfortunately you haven't got it."
The event, held on Sunday 1 March, was criticised by some runners after they found their personal best times were faster than expected.
St David's Hospice, which organised the race, said an error was made when the course was measured out.
Director Kris Broome said: "We sincerely hope that this news will not detract from the many positive aspects of the event... We hope to make the 2016 city of Newport half marathon even bigger and brighter."