Oct 20 (Reuters) - U.S. drugmaker AbbVie Inc said on Monday it has reached agreement with Shire Plc to officially call off its proposed $55 billion purchase of the Irish company, and will pay Shire a $1.64 billion breakup fee for walking away from the deal.

AbbVie, which last week said its board of directors had withdrawn its recommendation to proceed with the Shire purchase, blamed the unraveling on last month's rules changes by the U.S. Treasury Department aimed at curtailing a wave of such deals in which U.S. companies would redomicile overseas to take advantage of lower corporate tax rates in other countries.

In announcing termination of the agreement, AbbVie said the U.S. Treasury moves "re-interpreted longstanding tax principles in a uniquely selective manner designed specifically to destroy the financial benefits of these types of transactions." (Reporting by Bill Berkrot; Editing by Bernard Orr)