International Airlines Group (IAG) is ordering 18 Boeing 777-9 aircraft, plus 24 options, for British Airways.
They will be used to replace 14 Boeing 747-400 and four Boeing 777-200 between 2022 and 2025. Each aircraft will be fitted with 325 seats in four cabins.
British Airways' 777-9 will be powered by General Electric GE9X engines. The engine order includes a comprehensive maintenance package with GE.
Willie Walsh, IAG chief executive, said: "The new B777-9 is the world's most fuel efficient longhaul aircraft and will bring many benefits to British Airways' fleet. It's the ideal replacement for the Boeing 747 and its size and range will be an excellent fit for the airline's existing network.
"This aircraft will provide further cost efficiencies and environmental benefits with fuel cost per seat improvements of 30 per cent compared to the Boeing 747. It also provides an enhanced passenger experience".
British Airways has 135 wide-bodied longhaul aircraft in its fleet (12 A380s, 30 B787s, 12 B777-300ERs, 46 B777-200s and 35 B747s) with 12 more B787s, four B777-300ERs and 18 A350s on order.