The airline, whose alliance with American Airlines and Spain's Iberia is being scrutinised by European and U.S. competition watchdogs, on Friday said revenues fell 13.7 percent to 4.1 billion pounds in the six months to the end of September.

Its pretax loss -- impacted by higher debt levels, lower interest rates and a higher pension burden -- compares to the pretax profit of 52 million pounds it made in the same period last year.

The airline said it would look to drive more costs out of the business in the coming months.

"Aviation remains in recession ... with revenue likely to be 1 billion pounds lower this year, we can't stand still and further cost reduction is essential," Chief Executive Willie Walsh said in a statement.

The global recession has battered the airline industry as consumers cut back on trips abroad and lucrative business class travellers fly less. BA has also been hit by growing competition from low-cost carriers and potential labour strikes.

BA is expected to report a pretax loss of 568.76 million pounds for the year to the end of March, according to a Thomson Reuters I/B/E/S poll of 19 analysts.

Shares in BA, which have risen 27 percent in the last quarter, closed at 186.30 pence on Thursday, valuing the group at around 2.2 billion pounds.

(Reporting by Rhys Jones; editing by Julie Crust)