Construction of the outer shell, including roof sections, was finished Thursday, 14 months after work started and three years before the start of the games.
The first of the 85-tonne steel sections of the roof was lifted into place at the end of January 2009, and now all 28 sections are in place more than a month earlier than originally planned.
The 80,000-seat stadium will host the opening and closing ceremonies and track and field competition.
ODA chairman John Armitt said the Olympics will be delivered within the budget despite the "external shocks" caused the recession.
"We have hit all, and exceeded some, of the ten Milestones we set ourselves last year and we are on track and within budget."
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