CKE Restaurants Inc. on Tuesday said that its board plans to end a merger agreement with Thomas H. Lee Partners because it has determined that a later offer by another entity is superior.

The Carpinteria operator of the Carl’s Jr. and Hardee’s fast food chains said the $12.55 per share offer by the unidentified party is superior to an initial offer from affiliates of THL Partners, a Boston firm that co-owns Dunkin’ Brands Inc.

A CKE spokeswoman said the company can’t identify the new bidder at this time due to a confidentiality agreement. Bloomberg News is reporting that its sources say Apollo Management LP is the rival bidder.

Apollo is a private equity investment firm, founded by former Drexel Burnham Lambert banker Leon Black, which has headquarters in New York and significant operations in Los Angeles. Charles Zehren, an Apollo spokesman, declined comment on the Bloomberg report.

In February, THL Partners offered $11.05 in cash per share, a deal valued at the time at about $928 million since it included assumption of about $309 million in net debt. The agreement included a clause allowing CKE to consider superior offers until April 6.

CKE said it notified Lee on Monday that it plans to terminate its existing merger agreement, but is obligated to give THL Partners at least four days to make a counter-offer.

Shares were up 84 cents, or 7 percent, to $12.83 in midday trading on the New York Stock Exchange.