Organisers of the Brighton Half Marathon have apologised after it emerged the course has been 146 metres (479ft) short for the past three years.
An inquiry was launched after runners reported that data recorded on their tracking devices failed to measure up to the 13.1-mile distance.
UK Athletics contacted Brighton Half Marathon organisers following concerns about last month's race, and it has emerged the course had fallen short by 146m (0.09 miles) since 2015.
The area of concern centred on a turning point just after mile four where the course passes the independent Roedean School before turning and heading back towards Brighton.
In a statement, Brighton Half Marathon said: "We have concluded that the eastern turning point has been positioned incorrectly over the last three half marathons, resulting in a shortfall in the overall half marathon distance.
"We are devastated that this mistake has happened and apologise unreservedly to all runners who took part in the affected race years."
The news means that runners face having their personal bests, records and race times affected. And officials admitted the error was likely to have overshadowed the event for many.
DJ Norman Cook, aka Fatboy Slim, launched this year's marathon, marking the 27th year of the race, which raises funds for the Sussex Beacon, supporting an HIV care centre in Brighton.
Race director Martin Harrigan said: "We are a team of runners ourselves so we fully understand the impact of this news. We are really upset that this mistake has happened and we take full responsibility for this situation."
Simon Dowe, CEO of the Sussex Beacon, which organises the race, said: "We can't apologise enough for this mistake. I know it's hugely disappointing to everyone who trained so hard."