Oil refiners Idemitsu Kosan Co. and Showa Shell Sekiyu K.K. said Tuesday they have agreed to merge April 1, after Idemitsu's founding family dropped its long-standing opposition to the plan.

The refiners will merge via a share swap, and Showa Shell will be delisted March 29, they said in a statement, pushing the shares of the two companies up sharply in afternoon trading.

The combined firm would account for about 30 percent of gasoline sales in Japan, second only to JXTG Holdings, which controls about half the market.