The government's antitrust watchdog will soon draw a conclusion that a four-year payment plan for smartphones offered by two major phone carriers is legally problematic, sources close to the matter said Friday.

The Japan Fair Trade Commission believes the payment plans offered by KDDI Corp. and SoftBank Group Corp. for the iPhone and other mobile devices may deprive customers of options to choose other carriers, a potential violation of the antitrust law, according to the sources.

Working together with the communications ministry, the commission is expected to request that phone carriers take corrective measures.