Olympic Delivery Authority chief executive David Higgins told the Department for Culture, Media and Sport Committee: "A project of this size and scale with all the risk attached to it - hopefully that is a conservative statement. It is a prudent contingency at this stage in the project.
"We do not intend to go back to the Funders Committee for further allocations in the next six months - that's for certain."
ODA chairman John Armitt added: "You can say to me `do I guarantee that absolutely this is not going to happen' I cannot say that."
He described the approach as 'realistic' to outline this as 'the absolute maximum that we need for the Games'.
Last month Jonathan Stephens, the permanent secretary at the DCMS, conceded it was now realistic and prudent to expect the project's contingency to be spent in its entirety.
DCMS secretary James Purnell is to announce an update on spending of the overall £9.3billion Olympic budget, which has already trebled, to Parliament within two weeks.