(Bloomberg) Walgreen Co., the biggest U.S. drugstore chain, plans to announce it will buy all of Alliance Boots Holdings Ltd. and won’t use the deal to move its tax address abroad, said a person familiar with the matter.

Walgreen already owns 45 percent of Bern, Switzerland- based Boots, which has pharmacy and beauty stores in Europe. It will pay between 5 billion and 6 billion pounds ($8.44 billion to $10.1 billion) for the remaining stake, said the person. Walgreen had previously considered moving its headquarters as a way to lower the rate it pays the U.S. government. The person spoke on condition of anonymity because the matter isn’t public.

“A potential tax inversion has become a hot button topic in recent weeks as the company (and a handful of outspoken investors) have weighed the option,” said Ross Muken, an analyst with ISI Group LLC, in a note to clients. Moving overseas could have saved Walgreen at least $4 billion in tax savings over five years, he said.

Anjali Unnikrishnan, an outside spokeswoman for Boots with RLM Finsbury, declined to comment. Walgreen did not respond to a request for comment.

Walgreen shares fell 4.2 percent to $69.12 at 4 p.m. in New York, the biggest one-day drop in more than a year.

The U.S. pharmacy chain has come under political pressure not to do a so-called tax inversion as other health companies, including drugmakers AbbVie Inc. and Pfizer Inc., struck or attempted deals to cut their own rates and leave the U.S.

’Turning Your Backs’
Senator Richard Durbin, the Illinois Democrat, said in a July 22 letter to CEO Gregory Wasson that should the company decide to go through with a tax inversion, management would be “turning your backs on the very people that have allowed Walgreen’s to thrive and prosper.”

Lawmakers in both parties have examined ways to stop the departures. The Treasury Department said today it is examining whether it has the authority to stop companies moving overseas while it waits on Congress to pass comprehensive tax reform.

Walgreen took a 45 percent stake in Boots for $6.7 billion in 2012 in an effort to build a global pharmacy chain. Walgreen had the option to gain full control of the chain within three years of that purchase. The deal was reported by Sky News earlier today.

At 35 percent, the U.S. has the highest corporate tax rate in the developed world and is one of few countries that makes companies pay that rate on all the global income they bring home.