Kayak is an online travel information search business that aggregates other travel booking sites to provide feedback to the user. The priceline.com merger for $40 per share in cash and stock was announced Nov. 8 and approved by shareholders of Kayak on March 4. The deal cleared U.S. antitrust regulators Jan. 7. The companies anticipated closing the merger by the end of the first quarter. Priceline.com had requested that the U.K. Office of Fair Trading deem the merger not qualified for review but the OFT extended its review of the transaction last week anyway.

OFT is considering whether the arrangement may be expected to result in a "substantial lessening" of competition within any U.K. market.

Competition in the online travel business is fairly robust. Priceline and Kayak each compete with sites of Expedia Inc. and Orbitz.com as well as Travelocity.com Inc., Google Inc. and Microsoft Corp.

In December, Expedia acquired a 62% stake in Trivago GmbH of Germany for $630 million. Trivago is a well-established online search site in Europe that, like Kayak, aggregates other travel booking sites in its search process. The Trivago deal cleared the OFT March 4 after it was determined that it did not qualify for an extended review.

The OFT review of the Kayak merger is now set to expire on May 9.

The priceline.com 10-K does reference an investigation by the OFT that was launched in September 2010 but resulted in a statement of objections in July 2012 against priceline.com subsidiary Booking.com. The OFT alleged that Booking.com participated in actions that limited the ability of other, smaller online booking companies to offer room-only reservation services. But the OFT probe includes Expedia and the OFT said in July that while it limited the scope of its investigation to major companies, the alleged practices are potentially widespread in the industry. It is possible the extended review of Kayak is related to that investigation.

Priceline, in its 10-K, disputes the allegations and says that Booking.com's hotel reservation platform offers hotels complete discretion and control over setting prices and the site is not a reseller of hotel rooms. Priceline expects a final decision could come from the OFT in the second half of 2013.

Kayak and priceline.com did not return calls.

The extended OFT review does affect the trading in the Kayak deal. In the merger, Kayak shares are exchanged for $40 per share in cash or stock with a 33% cap on the cash portion of the consideration. The stock end of the deal also has a collar with a 30-day pricing period ending two days before the close. That pricing period would have been expected to have begun last month, but now will begin next week at the earliest.

Kayak shares traded Tuesday at $39.90, at a nominal premium of 9 cents, or 0.2%, to their deal value. But the exchange ratio for the stock portion of the deal will not be known until May.