FARNBOROUGH, England (Reuters) - Airbus and Boeing head into next week's Farnborough Airshow locked in their fiercest market share battle for up to a decade, slashing prices to win key orders for their latest narrowbody jets and storing up potential trouble for future profit margins.

From Australia to Indonesia, the United States to Norway - and most recently, Reuters has learned, Turkey - the marauding plane giants have spent months taking or defending market share, with world No.1 Airbus aiming deep inside Boeing's territory and its rival publicly vowing to defend a traditional 50-50 split.

While Airbus unveiled an assembly plant in Boeing's backyard this week, the battle behind the scenes - described as "hand-to-hand fighting" from airline to airline by one participant - has implications for the business case of the A320 jets to be built on U.S. soil as well as the profitability of Boeing's rival 737.

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Neither side can afford to let its share of this part of the market - estimated at $2 trillion over 20 years - drift far below the typical 45-55 percent band seen in a duopoly.

Doing so would let the other run away with efficiencies of scale and benefits of experience or learning curve. From there it is hard to recover as Douglas found when it lost independence through a 1997 merger between Boeing and McDonnell Douglas.