Priceline’s $1.8 Billion Kayak Delayed by Antitrust Probe Inc., the most valuable online-travel agency, said the $1.8 billion purchase of Kayak Software Corp. will close later than expected because of an antitrust review by U.K. regulators.

The U.K. Office of Fair Trading has a May deadline for its examination of the merger, the Norwalk, Connecticut-based company said in a statement today. The deal was originally expected to close by the end of the first quarter of 2013.

Priceline has been using acquisitions to add customers as it works to grow sales and fend off competition in a sluggish economy. Kayak, which raised $91 million in an initial public offering in July, lets travelers compare prices and make reservations for hotels, flights, cars and vacations.

Last month, Priceline said it was in the process of obtaining approval from European regulators and expected a vote on the transaction at a Kayak shareholder meeting planned for March.

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