Airbus has been seeking outside capital because it estimates the huge jet, dubbed the A3XX, would cost more than $8bn (pounds 5bn) to develop. Industry experts suggest the figure could top $12bn. Airbus hopes the super jumbo could eventually supplant the Boeing 747 as the world's premier large plane.
A spokesman for British Aerospace, which controls 20 per cent of Airbus, said: "The agreement provides for Alenia to draw nearer to Airbus. As part of the broadening evolution of the Airbus partnership it is expected that Alenia will be increasing its participation in Airbus strategically and organisationally."
Alenia, which is part of the Italian defence group Finmeccanica, is also expected to be involved in the development of a 100-seater regional jet which Airbus is building in co-operation with Singapore and China.
Alenia will continue its established participation in the Airbus military transport programme. It will also be asked to participate in the programmes of new derivatives from existing aircraft such as the A340-600.
Airbus, which is based in Toulouse in France, will this month produce a binding agreement on plans to re-shape itself as a stand-alone company by 1999. It is currently a loose confederation of manufacturers, a structure which analysts say leads to higher costs than arch-rival Boeing's.
Airbus is currently 38 per cent owned by Germany's Daimler Benz. Aerospatiale of France also has 38 per cent with 20 per cent held by the UK's BAe and 4 per cent by Casa of Spain.
Airbus has also indicated the possibility of Saab of Sweden becoming a potential partner. "We have an interest and we have been approached and we are discussing it," Lars Jagerfelt, a Saab spokesman, said. Airbus spokesman David Voskuhl said the company would not rule out adding Saab as a partner but said it was actively seeking additional partnerships world-wide.
In the first half of this year Boeing's stake of the world market for passenger jets with at least 100 seats was 57 per cent compared with Airbus' 37.7 per cent. Boeing's advantage is in jumbo jets which seat more than 400 passengers.