Northern Ireland-based Wrightbus will manufacture the hybrid buses as per the contract it won in 2009.
The order will be the largest order for hybrid buses across Europe with the first batch of 30 buses expected to be pressed into service in April 2013.
Technology to be used for the buses combines batteries, a conventional diesel engine and an electric motor to deliver 30% fuel efficiency and reducing carbon emissions by 20,600t per annum.
Commenting on the order Mayor of London Boris Johnson reminded that it was one of his election promises to expand the number of hybrid vehicles in the city and was hopeful that they will deliver significant environmental benefits.
London Transport Commissioner Peter Hendy added, "These new clean, green buses have proved a big hit with passengers who appreciate the smooth ride and spaciousness of the bus."
Complete delivery of the 600 buses will be completed by 2016, taking the then count of hybrid buses in the British capital to over a 1000.
