The 20,000 people who missed out on London 2012 tickets in the first two rounds of sales will be given priority when 900,000 more tickets go on sale on Friday.
From 11am, they will have 31 hours' exclusive access to the tickets before another one million people who were unsuccessful in the first ballot last year. They will then have a five-day sales period to try to buy tickets on a first-come, first-served basis.
Each person will be limited to applying for one session and a maximum of four tickets.
For the first time, 70,000 Olympic Park tickets will be sold which will allow access to the Park to watch sport on the big screens but not to any actual sessions. Those tickets will be £10 or £5 for those aged 16 or under or 60 or over.
The 20,000 people who missed out completely in the sales rounds in March and June last year will get first bite - and will have a chance of tickets for all events including a limited number for the opening and closing ceremonies.
They will be informed by organisers via email.
After that, the one million people who applied in the initial ballot but were unsuccessful will have tickets released for different sports over five days.
London 2012 chairman Lord Coe said: "We know thousands of sports fans were disappointed when they missed out in the initial sales period because of the massive demand for tickets.
"We promised we would prioritise these fans when we released the contingency tickets, which is exactly what we are doing."