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Airbus sales chief John Leahy said at the Paris Airshow on Wednesday that the A350 deal would likely be signed by the end of the month.
Sri-Lankan, which operates an all-Airbus fleet, said it also plans to lease three A350s, adding that a lessor had not yet been chosen.
Delivery of the new A330-300s is due to start next year, while the four A350s would be delivered between 2017 and 2023.
Engine maker Rolls-Royce said it won an $800 million order at list prices for Trent XWB engines to power the A350-900s and Trent 700 engines to power the A330-300s.