Games will act as a 'pressure point' to clean up east London
The Olympic Games site may be riddled with unexploded bombs dropped in World War II.
Over 200 of the 1500 bombs that were dropped in east London during the war failed to detonate according to a report by the London Development Agency (LDA) and could be buried under the Olympics site.
The report has classed the site as "high risk" and the area is being scanned for ordnance.
The cost of the work has been factored into the site clean-up.
A civil defence map used in the report shows where bombs fell over east London and says that of 1,493 bombs identified, 207 remain unexploded.
London Mayor Ken Livingstone said the Olympic site was one of the most polluted places in southern England.
"Of course there's a risk of bombs," he said. "We did get a lot of dropped on us during the war and we haven't found them all yet.
"The only way you're going to get this dealt with is with a real pressure point like the Olympics."