Organisers of the Brighton Half Marathon have apologised after it was revealed the course has been 146 metres (0.09 miles) short for the past three years.
An investigation was launched after British Athletics analysed GPS data.
"We're devastated that this mistake has happened," said the race organisers.
"We take full responsibility for this situation and apologise unreservedly to all runners who took part in the affected years."
It is the latest in a series of measuring errors which have affected high-profile mass participation races.
And it is not the first time Brighton organisers have had to apologise - after they revealed the course was too long in 2012.
Also, in 2016, the Great Scottish Run half-marathon course in Glasgow was found to be about 150m short after it was re-measured.
The error meant the record set by winner Callum Hawkins was invalid as were personal best times recorded by runners.
In addition, times from the 2013, 2014 and 2015 Greater Manchester Marathon were discounted, affecting about 24,000 runners, after the course was found to be 380m too short.