By Jonathan Stempel and Andrew Chung

NEW YORK (Reuters) - Citigroup Inc <C.N> and AT&T Inc <T.N> have ended a court battle over whether the "AT&T thanks" customer loyalty program infringed Citigroup's trademark in the phrase "thankyou."

According to a Monday filing with a Manhattan federal court, the companies have dropped claims against each other with prejudice, meaning they cannot be brought again.

Citigroup sued AT&T in June, calling "AT&T thanks" too similar to the "thankyou" that the New York-based bank had used since 2004 on its own customer programs.

AT&T countered that Citigroup has no monopoly over the word "thanks" and sought a court order to that effect.

The resolution may help preserve a relationship between Citigroup and AT&T dating to 1998 that includes 1.7 million U.S. customers with co-branded credit cards.

"We have decided not to pursue this matter any further and look forward to continuing to work with AT&T," Citigroup spokeswoman Jennifer Bombardier said in a statement.

AT&T spokesman Fletcher Cook said: "We consider the matter closed."

The case was dropped 11 days after U.S. District Judge Katherine Forrest in Manhattan rejected Citigroup's request for a preliminary injunction against "AT&T thanks."

She said Citigroup did not show that "AT&T thanks" would necessarily confuse customers or cause it irreparable harm, though "AT&T thanks" and "thankyou" share some letters and pronunciation, and both "convey a message of gratitude."

The case is Citigroup Inc v. AT&T Inc et al, U.S. District Court, Southern District of New York, No. 16-04333.

(Reporting by Andrew Chung and Jonathan Stempel in New York; Editing by Steve Orlofsky and Andrew Hay)