The deal is a major boost for developers British Land and Oxford Properties and further evidence of a strong recovery in the London office market.
Aon has agreed a 191,000 sq ft pre-let of the 610,000 sq ft Leadenhall Building.
The insurance firm will take the lower 10 floors of the offices (levels 4-13), and also has options to take up to a further 85,000 sq ft of space on levels 14-18.
Construction of the tower started in January with practical completion scheduled for the mid 2014.
Chris Grigg, Chief Executive of British Land said, “We are delighted that Aon has chosen The Leadenhall Building as the headquarters for its UK operations.
“This agreement demonstrates the high level of interest being shown by major occupiers for this iconic London office building and the world-class accommodation it will offer.”