* Founding Idemitsu family still opposes merger
* Purchase of stake from RDS delayed, awaiting FTC ruling (Adds background on merger plans)
TOKYO, Dec 19 (Reuters) - Japan's Fair Trade Commission said it would hold a news conference at 4 p.m. (0700 GMT) on Monday regarding its decision on whether to approve Idemitsu Kosan Co's planned takeover of rival refiner Showa Shell.
The deal has seen several setbacks with the Idemitsu founding family threatening to veto it once it is put up for shareholders' approval.
In the face of that opposition, Idemitsu management in October postponed a full takeover of Showa Shell indefinitely, aiming to bring the founding family on side.
But Idemitsu management has said it would go ahead with an agreed acquisition of a one-third stake in Showa Shell from Royal Dutch Shell once anti-trust authorities grant approval for the two companies to combine.
The acquisition of the stake had been scheduled to be completed by the end of September but has been delayed several times as the firms waited for an FTC ruling. (Reporting by Taiga Uranaka; Editing by Chang-Ran Kim and Edwina Gibbs)