"Nissan and Mitsubishi are discussing various matters including capital cooperation, but nothing has been decided," Nissan said on Thursday. The firms will hold a joint news conference later Thursday after separate board meetings.
The proposed deal would amount to Mitsubishi coming under the control of Nissan, making it the top shareholder ahead of Mitsubishi Heavy Industries, which holds a 20% stake, according to major media including national broadcaster NHK.
Mitsubishi admitted last month that unnamed employees had manipulated data to make cars seems more fuel-efficient than they were.
So far, Mitsubishi has confirmed that four models and more than 600,000 vehicles - all sold in Japan - were involved in the fuel cheating, but warned the number of cars affected would likely rise.
Analysts said a tie-up would give Nissan access to Mitsubishi's strong foothold in Southeast Asia and some key technology, including hybrids and minicars, which are hugely popular in Japan. The move would also help drive Nissan CEO Carlos Ghosn's long-held target of running one of the world's top three auto groups.