Allen & Overy (A&O) has taken the lead role on British Airways' $6bn (£3.9bn) order of 18 new Airbus A350 aircraft.

International Airlines Group (IAG), the parent company of BA and Iberia, has signed the agreement with French manufacturer Airbus as part of a fleet renewal and modernisation strategy. 

IAG said the deal included the option to buy a further 18, and will be used to replace 30 Boeing 747-400 aircraft between 2017 and 2023. 

A&O advised IAG, with finance partner Paul Nelson leading a team that included associate Chris Mitchell, working alongside Jeremy Moreton, BA's head of commercial law.

Nelson said: "We have been assisting throughout on this strategically important project, with considerable work involved in ensuring the transaction reached a successful close."

A&O's relationship with IAG dates back to its links with Iberia, which the magic circle firm has advised on most of its major financing projects since 2003.

Since the 2010 merger of BA and Iberia, A&O has taken several roles for the airline, including a $805m (£521m) revolving credit facility for BA, which signed last November. 

Airbus and Boeing used their own internal legal counsel.

IAG is also considering further Airbus or Boeing orders for Iberia following a recent restructuring of its cost base.