FEDERAL WAY, Wash., Dec 13 (Reuters) - Weyerhaeuser Co. on Thursday raised its offer to acquire smaller rival Willamette Inc. to $55 per share, or $6.05 billion, in cash, aiming to head off Willamette's efforts to acquire Georgia-Pacific Corp.'s building products unit.

Weyerhaeuser said the offer, which represents a 10 percent increase over its previous bid of $50 per share, represented its ``best and final'' offer. The Federal Way, Washington-based company also said it would withdraw its bid if Willamette signed an agreement to buy the Georgia Pacific unit.

Weyerhaeuser launched a hostile bid for Willamette more than a year ago, which was rejected. Earlier this year, Willamette offered to open negotiations if Weyerhaeuser offered to bid closer to $60 per share.