By Jonathan Stempel

NEW YORK (Reuters) - Citigroup Inc <C.N> sued AT&T Inc <T.N> on Friday, saying the phone company's use of "thanks" and "AT&T thanks" in a new customer loyalty program infringed its trademark rights to the phrase "thankyou."

The lawsuit may threaten the business relationship between two of the largest U.S. companies, which Citigroup said dates to at least 1998 when they launched the AT&T Universal Card.

In its complaint filed in the U.S. District Court in Manhattan, Citigroup said it has since 2004 used "thankyou" extensively in promoting its own customer loyalty and reward programs, including credit cards co-branded with AT&T.

But the New York-based bank, the nation's fourth-largest by assets, said the "AT&T thanks" program launched on June 2 will likely confuse consumers, and irreparably damaged its goodwill and reputation associated with the "thankyou" trademarks it uses for various banking services.

Citigroup also said Dallas-based AT&T asked the U.S. government in April to register an "AT&T thanks" trademark despite knowing the bank's concerns.

AT&T's infringement was "knowing, intentional, and willful," and forced Citigroup to sue to protect its rights, the complaint said.

The lawsuit seeks to stop AT&T from using the phrases "thanks" and "AT&T thanks" in its programs and marketing. It also seeks unspecified triple and punitive damages.

AT&T plans to defend against the lawsuit, spokesman Fletcher Cook said.

"This may come as a surprise to Citigroup, but the law does not allow one company to own the word 'thanks,'" Cook said. "We're going to continue to say thanks to our customers."

Liz Fogarty, a Citigroup spokeswoman, declined to provide additional comment.

Citigroup said its "thankyou" programs have about 15 million members in the United States, and that 1.7 million customers there have credit cards co-branded by Citigroup and AT&T.

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 Jonathan Stempel in New York; Editing by W Simon and Tom Brown)