By Erwin Seba and Diane Bartz

HOUSTON/WASHINGTON (Reuters) - Independent western U.S. refiner Tesoro Corp may take ownership of BP Plc's 240,000 barrel per day (bpd) refinery in Carson, California, as early as June 1, sources familiar with the transaction said on Thursday.

Other sources told Reuters that the U.S. Federal Trade Commission, which assessed the deal to ensure that it complied with antitrust law, were prepared to approve the purchase within days.

That approval could come as early as Friday, said one source with knowledge of discussions between the company and the agency. It was not known if the FTC will place conditions on the deal's approval.

The sources could not speak for attribution, citing the need to protect business relationships. A BP spokesman declined to discuss the transaction.

Tesoro announced last August that it had agreed to buy BP's Carson plant for $2.5 billion. The proposed deal has been awaiting approval by the FTC and by California's attorney general.

The sale includes an 800-station retail network and distribution and storage assets.

Tesoro is the second-largest refiner in California after Chevron Corp . California is the largest gasoline market in the United States.

In recent days, owners of BP Arco-branded retail stations in California have been notified to expect the change to Tesoro as early as the first day of June. "June 1, that's the date for Tesoro to take over," one of the sources said.

Once the transaction is cleared, BP's U.S. downstream operation will be solely focused on refineries in the northern continental United States, where cheaper Canadian crude oil is easily obtained.

A Tesoro spokeswoman declined to discuss the status of the deal.

"As previously communicated, we expect the transaction to close before mid-2013," said Tina Barbee, who declined to elaborate.

An FTC spokesman declined on Thursday to discuss the status of the commission's review of the transaction. On May 2, Tesoro Chief Executive Greg Goff said the company was near the end of the regulatory review process.

A spokeswoman for California Attorney General Kamala Harris did not have an immediate comment about the pending sale.

In addition to the refinery and retail network, the sale includes more than 100 miles of pipeline, three marine terminals, four land storage terminals and four product marketing terminals.

Tesoro plans to sell the distribution and storage assets to its master limited partnership, Tesoro Logistics LP , for about $1 billion, within a year of closing.

Tesoro plans to combine operations of the Carson refinery with its 103,800 bpd refinery in Wilmington, California. The two refineries nearly about one another in the Los Angeles industrial suburbs north of the port of Long Beach.

BP announced plans to sell the Carson refinery as well as its Texas City, Texas, refinery in 2011. Marathon Oil Corp purchased the Texas refinery in February as part of a $2.4 billion deal that includes terminals, pipelines and other assets.

(Reporting by Erwin Seba in Houston and Diane Bartz in Washington; editing by Gerald E. McCormick, Tim Dobbyn and Matthew Lewis)

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