Fedex has reached an agreement to purchase another 50 Boeing 767-300 freighters worth US$9.97 billion at current list prices.

This is the biggest order ever for the aircraft, allowing the plane manufacturer to extend its production line well into the next decade. The deal includes options for another fifty 767Fs.

FedEx said the aircraft will be delivered by Boeing for its FedEx Express arm over the fiscal years 2018-2023. The latest deal brings FedEx's firm orders for 767Fs to 106, reported Reuters.

The 767Fs will replace older freighters, including Boeing MD10s and MD11s as well as Airbus A300-600s and A310s, which the express delivery giant plans to retire in the coming years.

"Acquiring additional 767F aircraft will enable us to reduce structural costs, improve our fuel efficiency and enhance the reliability of our global network," said FedEx CEO David Bronczek.

FedEx had a fleet of 641 freighter aircraft as of May 31, according to data on its website. In addition to the 767F orders, it has also committed to 18 Boeing 777 freighters.

Boeing earlier revealed that Taiwan's EVA Air has finalised an order for five 777 freighters worth $1.5 billion.