NEW YORK, March 17 (Reuters) - BP will block around 800 workers from its 440,000 barrel-per-day Whiting, Indiana, refinery starting March 19, the British oil major said on Tuesday, as months of contract negotiations yielded little results.

The labor dispute at Whiting, the largest refinery in the Midwest, came at a time of high gasoline and diesel prices as the Middle East war upends global energy supplies. Any operational disruptions could exacerbate an already tight fuel market, sending prices even higher.

The United Steelworkers union represents about 800 workers at the Whiting refinery that produces key transportation fuels including gasoline, diesel fuel and jet fuel.

BP on Tuesday issued a lockout notice after the United Steelworkers-represented workers rejected its "last, best and final" offer.

The company has told maintenance workers at the Whiting plant to not report for work after Tuesday, and other union-represented workers to work through March 18.

Last week, BP presented a revised contract offer after union members overwhelmingly rejected what the company its final proposal.

"In today's meeting, the Union offered a slightly revised proposal, but once again unequivocally rejected the Company's proposals that are critical to the long-term sustainability of the Refinery," BP said in a statement.

United Steelworkers was not immediately available for comment.

The company noted it would continue bargaining in good faith, but lifting the lockout would require the union's acceptance of its March 17 proposal.

BP said the decision to impose a lockout followed months of negotiations during which the union twice rejected key proposals without addressing BP's primary concerns.

Since the previous three-year collective bargaining agreement expired on January 31, the company said it has been operating amid "labor uncertainty," including the possibility of a strike with as little as 24 hours' notice, and regaining operational control was vital to ensure a safe and orderly transfer of refinery management.

The union had said BP proposed workplace changes, including cutting more than 200 union jobs in operations, maintenance and environmental safety as well as stripping away some workplace protections.