Airbus is battling rival Boeing to be the biggest commercial planemaker

The move comes after BAE and EADS, which owns 80% of Airbus, failed to reach agreement on the price.

This means that investment banks for the two firms will have two weeks to negotiate the deal. If they fail, an independent bank will set the price.

The Airbus stake is worth �3bn-�4.5bn ($5.3bn-$8.4bn), analysts have said.

A "put" option gives a party the right to sell something to a second party, often within a certain period of time.

In this case, it means that EADS, the Franco-German firm which includes both commercial and military aviation, is obliged to buy BAE's Airbus stake.

Trigger point

The "put" option is part of a shareholders' agreement which was drawn up in 2001.

Under the terms of the deal, BAE can call in independent negotiators - in this case, one merchant bank that has been chosen by BAE and EADS - to agree on a sale price for the Airbus stake.

A spokesman for BAE said that the agreement ensured that the stake could be sold in a timely manner.

AIRBUS KEY FACTS The first ever Airbus, an A300, took off in October 1974 British Aerospace joined the consortium in 1979 There are over 3,900 Airbus jets in service Airbus employs a total of 55,000 staff Worldwide Airbus turnover in 2005 was �15.3bn

Analysts said that BAE may be looking to invest more money in its fast-growing US operations, where it produces 35% of its sales.

BAE said that it was looking to maximise its investment in Airbus in the best interests of shareholders.

Despite a bumper year for Airbus sales in 2005, profits at BAE's commercial aerospace division fell to �179m from �201m a year earlier.

BAE's move towards concentrating on its defence operations was reflected by last year's acquisition of US firm United Defense for �2.1bn.