Nirmal Kotecha was brought into the agency as major projects director three years ago to bring a more commercial approach after Keith Miller left following a critical report into project delivery.
Over this time Kotecha has revamped project control systems and built strong relations with the agency’s contractors and consultants.
Kotecha is understood to be leaving to join energy distributor UK Power Networks, which is planning a huge invest programme into power line replacement across the south east.
An agency spokesman said: “He has worked with his team of around 350 staff to put in place effective project control systems that are recognised as best practice.
“Nirmal will continue in his role until mid October, while also preparing for the hand over to his successor.”