By Trisha Roy

(Reuters) - Walgreens Boots Alliance Inc <WBA.O> said on Monday its biggest shareholder, Stefano Pessina, will step down as the chief executive officer after five years at the helm, during which the pharmacy group's value more than halved.

Pessina, 79, appointed as the CEO in 2015 following the merger between Walgreens and Alliance Boots, would assume the role of executive chairman once a new CEO is appointed, the company said.

Walgreens has been struggling with disappointing performance in its Boots UK unit, stiff retail competition, especially from Amazon.com Inc <AMZN.O>, and reimbursement pressures, largely due to low generic drug prices.

The COVID-19 pandemic has further added to the woes of the company, which had to resort to store closures, job cuts and suspension of share repurchases at Boots.

Bringing in a new CEO makes sense as Walgreen's stock price has lagged over the last five years due to changes in the pharmacy landscape and vertical consolidation in U.S. healthcare services squeezing both growth and margins at the company, Evercore ISI analyst Elizabeth Anderson said.

"We would have preferred this announcement to be accompanied by an update on a chosen new CEO, but believe the company should cast a wide net to bring in fresh perspectives."

Shares of the Deerfield, Illinois company were down 3% at $39.28 in early trade.

"I look forward to continuing to serve the company as executive chairman, and to helping to ensure the success of the new chief executive officer in every way possible," said Pessina, who owns 16% stake in Walgreens, according to Refinitiv data.

Under Pessina, the company has https://reut.rs/332EmmT explored several strategic partnerships and acquisitions including whether to go private following private equity interest.

Executive chairman James Skinner will step down, but will remain on the board, Walgreens added.

(Reporting by Trisha Roy in Bengaluru; Editing by Shinjini Ganguli)