London 2012 organisers have admitted an error has led to 10,000 too many tickets being sold for synchronised swimming sessions at this summer's Olympics.
Around 3,000 people have been contacted by London 2012 and are being offered tickets for alternative events they had applied for - including athletics.
A London 2012 spokesperson said: "As a result of finalising the seating configurations in our venues and reconciling the millions of Olympic and Paralympic ticket orders against the seating plans for around 1,000 sporting sessions, we have discovered an error in seats available in four synchronised swimming sessions.
"In December we contacted around 3,000 customers who had applied for tickets in the four sessions during the second round sales process.
"We are exchanging their synchronised swimming tickets for tickets in other sports that they originally applied for."
A human error is believed to be responsible for 20,000 tickets being put on sale in the second round of sales rather than 10,000.
The alternative tickets will come from the pot of one million for all events that are due to go on sale in May once the seating configuration has been finalised at all the venues.
The process for people to resell any unwanted tickets in their possession opens on Friday.