MANILA, Jan 5 (Reuters) - The Philippine power grid operator, partly owned by China's State Grid Corp, is in hot water for what the government said was its failure to prevent an outage that left millions without electricity for days.

President Ferdinand Marcos Jr shared his dismay on Friday, saying the situation "caused significant hardship to our people, crippling businesses, compromising livelihoods and endangering those in need of health care."

Power supply was fully restored on Friday in affected areas that included the Panay island and parts of Western Visayas in the central Philippines.

China's State Grid has a 40% stake in the National Grid Corporation of the Philippines (NGCP) after winning a 25-year concession in 2007 to run the country's sole power transmission operator with other investors.

The remaining 60% is held by Filipino investors including Synergy Grid & Development Phils Inc., led by Henry Sy Jr., who belongs to the country's richest family, and insurance industry tycoon Robert Coyiuto Jr.

Marcos, in a video message posted on social media platforms, said NGCP failed to resort to manual load dropping, or rotational brownouts, resulting in the power crisis, a charge which the transmission company rejected.

"Accountability lies with the NGCP. They are tasked with grid stability. Stability involves proactive responses to breakdowns and unexpected events, a duty that NGCP unfortunately has not fulfilled adequately," Marcos said.

NGCP said in a statement on Friday the problem arose from unplanned shutdowns of power generators.

"We firmly refute allegations suggesting that NGCP failed in its obligation to stabilize the transmission system. We also take exception to the allegations that we were less than transparent in providing information to the public," NGCP said. (Reporting by Karen Lema and Mihail Flores; Editing by Andrew Cawthorne)