O’Rourke started on the £318m project in August after beating Skanska, Balfour Beatty, Leand Lease, Mace and McAlpine to win the deal from joint developer British Land and Oxford Properties.

British Land said: “We successfully appointed Laing O’Rourke as the main contractor in August 2011, within budget and programme and they are on site with the anticipated completion date moved ahead from Q3 2014 to Q2 2014.”

The project was also boosted today by the news that Aon has signed a deal to take 191,000 sq ft of space across the lower ten floors of the completed building.

Chris Grigg, Chief Executive of British Land said, “This significant letting to a leading global business is good news for the City and demonstrates the success of our commitment to creating well located world class buildings that provide long term value to occupiers.

“I am very pleased to welcome Aon to The Leadenhall Building and continuing our well established relationship with them.”