Organisers of a half marathon on England's south coast said they were "devastated" and apologised after it emerged the course had been 146 metres short for the past three years.
UK Athletics contacted Brighton Half Marathon organisers after runners reported their personal tracking devices showed they had not run the full 13.1-mile (21.1-kilometre) distance.
An inquiry found the course had somehow lost 146 metres of its length in 2015, prompting annoyed reactions from runners who felt their times for the race were meaningless.
"We are devastated that this mistake has happened and apologise unreservedly to all runners who took part in the affected race years," organisers said in a statement released on Saturday.
Eleanor Davis, who won this year's elite women's race in February, said: "The reason I ran the race was specifically because there was a deadline to get a qualifying time for the London Marathon.
"To find out afterwards that it counted for nothing was pretty devastating and concerning. But luckily the London Marathon have been really kind and waived it for me so I can compete."