LOS ANGELES (Reuters) - Striking harbor clerks reached a tentative deal with management at the twin ports of Los Angeles and Long Beach on Tuesday night, settling an eight-day labor clash that has idled most of America's biggest cargo-shipping complex.

The accord followed a resumption of talks with last-minute help from federal mediators and Los Angeles Mayor Anthony Villagraigosa, who announced the deal moments after union negotiators voted to approve it.

Officials for the International Longshore and Warehouse Workers (ILWU) Local 63 said the hundreds of striking clerical employees who walked off the job last Tuesday, and the thousands of longshoremen who refused to cross their picket lines, would return to work starting Wednesday morning.

Union officials also said they expected the rank and file to ratify the agreement. Details of the pact were not made public. But union spokesman Craig Merrilees said the deal was clinched by terms he said addressed workers' concerns about outsourcing.

With mounting economic losses estimated at several billion dollars, the strike is the largest cargo traffic disruption at the Southern California harbor facilities since a 10-day lockout of longshoremen at several West Coast ports in 2002.

The latest strike prompted at least 18 freighters to change course and take their cargo to other ports in Northern California, Mexico and Panama, according to the non-profit Maritime Exchange of Southern California, which tracks shipping in the region.

(Reporting by Dan Whitcomb,; Writing and additional reporting by Steve Gorman; editing by Patrick Graham)