(Adds wildfire warning details, high wind forecasts; byline)

By Steve Gorman

LOS ANGELES, Oct 8 (Reuters) - Nearly 800,000 California homes and businesses can expect to lose electricity for up to several days from Wednesday, in a planned PG&E power shutdown of unprecedented scale due to heightened wildfire risks from high winds, the utility said.

Pacific Gas & Electric Co, a unit of PG&E Corp, said on Tuesday it was extending a previously announced "public safety power shutoff" to 34 counties in northern and central California, marking the largest such precautionary outage the utility has undertaken to date.

Power will be turned off to communities in stages, starting in northern California, just after 12:01 a.m. Wednesday, depending on the timing of high-wind conditions locally, PG&E said in a statement.

Sustained gale-force winds were expected to last through midday Thursday, with isolated gusts of up to 70 miles per hour, the utility said. Once power is turned off, it cannot be restored until winds subside, allowing PG&E to inspect equipment for damage and make any repairs, it said.

"We're telling customers to be prepared for an outage that could last several days," PG&E spokeswoman Tamar Sarkissian told Reuters.

PG&E has come under increased scrutiny in recent years over maintenance of transmission wires and other equipment implicated in a number of major wildfires.

In May, state fire investigators determined that PG&E transmission lines caused the deadliest and most destructive wildfire on record in California, the wind-driven Camp Fire that killed 85 people in and around the town of Paradise last year.

Cal Fire likewise concluded that PG&E power lines had sparked a separate flurry of wildfires that swept California's wine country north of San Francisco Bay in 2017.

PG&E filed for bankruptcy in January 2019, citing potential civil liabilities in excess of $30 billion from the North Bay and Camp Fires.

RED-FLAG WARNING

The California Department of Forestry and Fire Protection (Cal Fire) said "red-flag" warnings were posted across the entire state for what was shaping up to be the strongest wind event so far this season.

The scope of the planned outage, extending to over half of California's 58 counties, will far exceed the very first public safety power shutoff declared by PG&E in October of last year, which impacted 60,000 customers, Sarkissian said.

Some consumer advocates have objected to the precautionary disruptions, saying they can harm people who need electricity for medical equipment.