The Tokyo 2020 Olympics will cost £9.98bn (1.8 trillion yen), six times more than the original estimate, it has been claimed.
Japanese public broadcaster NHK reported the overspend on its website.
It said the huge rise had been caused by unexpected payments for building lanes on highways, as well as rising material costs and personnel expenses.
However, if the cost of the Tokyo Olympics was to reach £9.98bn, it would still be considerably less than the 2014 Winter Olympics in Sochi, Russia, which was estimated to have cost £31bn ($51bn).
The initial cost of the 2020 Games was estimated at £1.6bn (300bn yen).
NHK said the figure was optimistic and the Tokyo 2020 Organising Committee was expected to show a shortfall of £5.54bn (1 trillion yen).
Total costs for staging the Games will be shared by the committee, the Tokyo government and national government.