BAE Systems has said it may call for an independent valuation of its 20% stake in aeroplane firm Airbus unless a sale price can be agreed with partner EADS.
Airbus is battling rival Boeing to be the biggest commercial planemaker
Defence firm BAE has been in talks with EADS, which owns 80% of Airbus, for three weeks and has said they have until the end of May to reach a deal.
Should they fail to do so, then BAE could exercise a "put" option, allowing it have the stake valued and then sold.
The Airbus stake is worth close to £3bn ($5.3bn), 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.
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
Some reports have suggested that BAE is asking too high a price, while EADS is looking to pay too little.
Both companies said that this was normal in negotiations, and reiterated that they were committed to finding a compromise.
"We are willing to pay a fair price," said Tome Enders, co-chief executive of EADS.
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.