Saying thanks costs nothing, as the adage goes. But not according to Citigroup, the US bank, which is suing AT&T on the grounds that its use of the term amounts to a trademark infringement.

For the past 12 years Citi has run a series of credit card loyalty schemes based on the “thankyou” brand, which awards points to users based on how much they spend. Last April AT&T, the phone company, became one of Citi’s partners in that programme, launching a co-branded card with a sign-on bonus covering up to the $650 of the cost of a phone.

But this month AT&T broke ranks, according to Citi, by launching a separate reward programme using the words “thanks” and “AT&T thanks”. In a complaint filed in a Manhattan federal court on Friday, Citi said that it was seeking injunctive relief and unspecified damages. It cited eight trademarks using the term “thankyou,” which it said was “conclusive” evidence of its sole right to use the marks on cards in the US.