WASHINGTON, Aug. 23 (AP) — Two major airlines were fined $300 million each Thursday after admitting that they had conspired to fix prices on international flights and agreeing to help prosecutors investigate other airlines.

The airlines — British Airways, Britain’s largest airline, and Korean Air, South Korea’s national carrier — pleaded guilty to antitrust conspiracy charges. They acknowledged colluding with rivals over cargo rates and fuel surcharges, which had been added to fares in response to rising oil prices. That meant higher costs for international shippers and passengers.

Judge John D. Bates of Federal District Court in Washington said the case “involved considerable commerce and reflected long-term and widespread conduct involving major airlines and players.”

Fines were reduced for both airlines because they had cooperated with Justice Department investigators. Korean Air’s fines could have been twice as high, and British Airways could have faced fines closer to $900 million.

“Any anticompetitive behavior is to be condemned at British Airways or at other companies. It will not be tolerated and we remain vigilant in this respect,” British Airways’ chief executive, Willie Walsh, said in a statement released after the court proceedings.

Korean Air released a statement saying it was “committed to antitrust compliance” and was taking steps to make sure the conduct was never repeated. Neither company addressed the extent of its cooperation or how widespread the practice was in the industry.

This month, authorities in London announced $246 million in fines for British Airways in a parallel trans-Atlantic investigation. Both inquiries continue.

Other companies were not named. Virgin Atlantic has been identified in the London investigation but is not expected to face a fine there because it reported its misconduct to authorities.

From 2004 to 2006, fuel surcharges rose from about $10 to about $120 a ticket for a round-trip, long-haul flight on B.A. or Virgin.

Judge Bates said that both British Airways and Korean Air had turned over hundreds of thousands of documents, some of which needed to be translated, and had made executives available for interviews with investigators. As foreign companies, they could have challenged the investigation and refused to cooperate.