By Joann S. Lublin and Paul Ziobro 

A small shareholder has launched a fight against a Walgreen Co. takeover backed by the company and activist investor Jana Partners LLC.

CtW Investment Group, an arm of labor federation Change to Win, wants stockholders of the biggest U.S. pharmacy chain to vote against Walgreen's roughly $14.7 billion plan to buy the 55% of European drugstore chain Alliance Boots GmbH it doesn't already own.

The group--which represents union pension funds that own about 2.5 million shares of Walgreen, or about 0.3% of the company--argued in a letter sent Wednesday to nearly all other shareholders that the acquisition is overpriced and unnecessary. Shareholders are to vote on the deal at a Dec. 29 special meeting.

CtW's letter also criticized Jana founder and managing partner Barry Rosenstein, who sits on Walgreen's board. "Mr. Rosenstein himself does not appear to bring either corporate integration, retail or international operating experience to the board," the letter said.

Jana declined to comment about the CtW letter.

Walgreen spokesman Michael Polzin said CtW has been opposed to the deal with Alliance Boots for some time, and that based on feedback from investors, the group "appears to be a significant outlier."

"Investors have consistently expressed support for the transaction and many have encouraged us to close this transaction as soon as possible," Mr. Polzin said.

In announcing the deal in August, Walgreen said buying the rest of Alliance Boots would create a global pharmacy operator better able to address rising drug costs.

CtW's objection highlights how the wave of mergers and activist-inspired strategic moves can sometimes leave smaller shareholders behind. Analysts generally favor the Alliance Boots deal, though some have said the outlook for the combined business isn't as good as it once was amid a slowdown in Europe and problems at Walgreen managing rising drug prices.

Alliance Boots operates about 4,500 pharmacies in more than two dozen countries. In 2012, Walgreen paid $6.7 billion in cash and stock for 45% of the European company in a move aimed at giving the company greater heft in negotiating with suppliers and a broader global presence. The company intended to buy the rest of Alliance Boots eventually, but sped up the timetable amid pressure from shareholders like Jana.

CtW argued in its letter that the existing partnership gives Walgreen the benefits of greater buying power without the cost of the full acquisition.

Jana holds a 1.4% stake in Walgreen and gained its board seat plus a promised second one in September. The activist fund supports the acquisition of the rest of Alliance Boots and had lobbied for Walgreen to relocate overseas as part of the process to save on taxes, people familiar with the matter have said, but the company decided not to amid a broader political backlash against such maneuvers.

Jana also supported Walgreen's decision in August to begin a $3 billion share buyback--a move that CtW considers unwise at a time that the company is already spending heavily on the acquisition. CtW will make the stock buyback "a central piece" of its campaign against the takeover vote, said Michael Pryce-Jones, its senior governance policy analyst.

David Benoit contributed to this article.

Write to Joann S. Lublin at joann.lublin@wsj.com and Paul Ziobro at Paul.Ziobro@wsj.com

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