Unexploded World War II bombs may be buried under the east London 2012 Olympic Games site, BBC London has learned.
More than 200 unexploded bombs are still buried in east London
More than 200 of the 1,500 bombs dropped in east London failed to detonate, said a report for the London Development Agency (LDA).
It classed some of the 500-acre site as "high risk" and experts have begun scanning the area for ordnance.
The cost of the work has been factored into the site clean-up, said the LDA.
At the peak of the Blitz thousands of bombs were dropped on London by the German Luftwaffe and some failed to explode.
A civil defence map used in the report, by independent site investigation firm Zetica, shows where bombs fell over east London.
Of 1,493 high explosive bombs identified, 207 remain unexploded, according to the report.
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 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."
Report author Mike Sainsbury, of Zetica, said the area was targeted by the Germans as it was close to east London docks.
He said: "I believe it was Hitler's own personal goal to strangle London by targeting docks."
Steve Davis, of the LDA, said nothing was being left to chance.
"We will be able to detect ordnance if it's there in the areas we're going to probe," he said.