The result of the race to host the 2012 Olympics Games is to be beamed live on giant screens in Trafalgar Square.
Dame Kelly Holmes will be in the crowd watching on 6 July
Dame Kelly Holmes will be among those watching the announcement of the International Olympic Committee decision from Singapore on 6 July.
"The race to host the Games is very close and I look forward to being part of the crowd that day", she said.
The "Thank You London, Thank You UK" event will feature live music and a performance of the bid anthem "Proud".
Dame Kelly said: "Whatever the outcome, London's bid has been a fantastic campaign that has really got the people of London and the whole of the UK involved and resulted in lots of benefits for sport and the country.
"As a nation we are passionate about sport, and I know we could host a fantastic Games. I really look forward to being part of the crowd on the day."
Sebastian Coe, London 2012 Chairman, said it was a great opportunity to say thank you to everyone who has supported the bid.
"Their backing has given us a real chance of winning," he added.