Credit rating firm Standard & Poor's said Monday it has put Hankyu Holdings Inc. on its watch list because the railway operator's planned merger with rival Hanshin Electric Railway Co. might prove too costly.

If the deal goes through, Hankyu's ratings -- BB for long-term corporate credit and BB-plus for senior unsecured debt -- could be downgraded, S&P said.

Hankyu formally announced April 28 that it is in merger talks with Hanshin, an integration premised on Hankyu's purchasing the 45 percent stake in Hanshin now held by an investment fund led by Yoshiaki Murakami.