The move would put pressure on Airbus Industrie, the European consortium in which British Aerospace is a partner, to invest hundreds of millions of pounds developing its own 550-seater carrier.
The airlines are due to meet Boeing again in April to iron out their requirement and configurations. Singapore Airlines is also expected to attend.
Analysts estimate that Boeing may have to spend up to pounds 666m developing a new wing to get the bigger 747 off the ground. Each aircraft would cost well over pounds 100m.
A BA spokesman said yesterday that the airline "fully expected to be in the vanguard of ordering this equipment", though he could not say when further talks with Boeing would be held. Lufthansa said it had held "intensive talks" with Boeing several weeks ago and that further talks on setting cabin size and layout were likely.
Boeing has always said development of what would be the world's largest passenger airliner, called the 747-600X, would be market-driven. Although BA was always tipped as a likely purchaser, Boeing needs launch orders from other airlines to make development worthwhile.
A Boeing spokesman said: "Certainly these airlines have been helping us and will continue to do so as we define a possible aircraft. This process is needed to find a consensus."
This week's edition of the industry magazine, Flight International, says that the 600X is expected to be launched in December, followed within six months by a longer-range 747-500X.
Boeing had been expected to develop a bigger 747 since abandoning talks with Airbus last year to develop jointly a 600-800-seater super jumbo.
Boeing is making most of its money selling larger aircraft where it has little competition, which is why most analysts believe Airbus will inevitably have to invest in a bigger product.Reuse content