One of Long Island MacArthur Airport’s new airlines is delaying its entry into the Islip market, citing an inability to acquire at least one Boeing 757 airplane for its planned round-trip routes to Puerto Rico.

National Airlines had previously said it would start flying its six weekly round-trip routes to Puerto Rico on June 1. The Town of Islip, which owns and operates the airport, sent out a news release Monday saying service would have to be delayed, and that passengers who have already bought tickets to Puerto Rico from Long Island will be refunded.

“National has experienced challenges acquiring the very popular Boeing 757 aircraft,” said Islip Town Supervisor Angie Carpenter in a statement. “Regretfully, this has prompted National to postpone their June 1 launch from Long Island MacArthur Airport to Puerto Rico. However, the Town remains very enthusiastic in welcoming National Airlines to our airport family.”

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Florida-based National currently operates a fleet of two Boeing 757 aircraft for its scheduled passenger operations. Airline representatives would not say how many airplanes National is hoping to acquire, how many ticketed passengers will be affected by the delay in launching service at MacArthur, or when the service would start. A spokeswoman said the airline is “constantly evaluating fleet deployment as it grows its business and the goal is for that to happen later this year.”

When the airline originally announced its nonstop round-trip service from Ronkonkoma to San Juan and Aguadilla in February, National Airlines said flights would start July 1. It moved that start date up to June 1 because of demand, the airline said at the time. They are planning four weekly roundtrips to San Juan and two weekly roundtrips to Aguadilla, leaving MacArthur in the morning and returning in the afternoon.

“This was not our choice, but rather due to the protracted negotiations for new aircraft making a June 1 start impossible,” said National Airlines president and chief operating officer Edward Davidson in a statement. “We continue to experience strong demand in our scheduled passenger service markets, but are taking a very measured approach to growing this part of our business. While we evaluate options to expand our aircraft fleet and identify potential markets for future expansion, we remain committed to serving our customers.”

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National is one of two airlines to announce new service at MacArthur this year: Maine-based Elite Airways will start providing twice-weekly flights to Portland, Maine; Myrtle Beach, South Carolina; and Melbourne, Florida, on June 17. Together, National and Elite bring the total number of airlines at MacArthur to four.

The airline said reservations for National’s routes to San Juan and Aguadilla are no longer being accepted, and customers who have already bought tickets “will receive a full refund to their original form of payment from the escrow account.” Customers with questions can call 855-757-6999.