The Olympic Stadium will retain the track and field facility
Plans for the use of the Olympic Park venue after 2012 are "still unclear", a London Assembly report has said.
The "continuing uncertainty" could make the venue "an island of prosperity, cut off from the surrounding community".
The report also said if a football club took over the main stadium it "may affect the character of the park and its surroundings".
The Olympic Park Legacy Company (OPLC) said finding long-term uses for the venues was a "fundamental aim".
The report by the Economic Development, Culture, Sport and Tourism Committee said: "The media centre and the Olympic Stadium are potentially the most important venues in terms of attracting jobs and investment to the park.
"The continuing uncertainty about the future of the main stadium undermines the potential to plan for legacy ahead of the 2012 Games.
"For example, the arrival of a Premier League football team in the middle of the Olympic Park may affect the character of the park and its surroundings."
'Time running out'
The report added that the OPLC needed to focus on skills, employment, sports participation and housing to benefit the deprived local community.
"It is important that the taxpayers see a return on the investment made by the Olympic Delivery Authority, and also that the village dovetails into the wider housing strategy for the park and the surrounding area," the report said.
Warning that "time is running out", the committee's chair Dee Doocey said: "A strong legacy of skills, housing, employment and sports participation is the only way to stop the Olympic Park becoming an island of prosperity, cut off from the surrounding community."
An OPLC spokeswoman said: "We welcome this report and the committee's ongoing commitment to work with us to create a lasting 2012 legacy.
"The development of a new metropolitan area that is integrated with its surroundings and provides employment and sporting opportunities for local people, along with securing a viable legacy use for the Olympic venues, are among the fundamental aims of the company which we are laying down in our corporate plan."
London 2012 chair Lord Coe has maintained that the stadium will retain its "track and field facility".