MADRID- The banking foundation that is the main shareholder in Naturgy has decided to increase its stake in the Spanish gas and power group ahead of an expected government decision on a 5 billion-euro partial takeover bid by an Australian fund.

Holding company CriteriaCaixa, which owns Caixabank, said on Tuesday its board would "reinforce its position" in Naturgy, of which it currently owns 24.8%.

CriteriaCaixa will not increase its stake beyond the 30% threshold that would require it to launch a tender offer for the entire company, it said in a statement.

Naturgy plans to shift away from gas towards renewable energy sources as part of a global drive to cut planet-warming emissions but is holding off presenting details to the market until Madrid decides whether to allow Australia's IFM Investors to buy a 22.7% stake.

Shares in Naturgy barely changed after CriteriaCaixa's statement. They soared close to the offer price when IFM made its original bid and are now changing hands for 21.8 euros each, just below the 22.37 euros IFM is offering. ($1 = 0.8192 euros)

(Reporting by Isla Binnie; Editing by Steve Orlofsky) ((isla.binnie@thomsonreuters.com; +39 06 8522 4392; Reuters Messaging: isla.binnie.thomsonreuters.com@reuters.net))