McDonald's Corp. (MCD) on Wednesday said President and Chief Operating Officer Mike Roberts, who the company had credited with revitalizing its U.S. business, resigned suddenly after 29 years with the hamburger chain.

Ralph Alvarez, 51, the head of McDonald's North American business, will succeed Roberts effective immediately, the company said. Don Thompson, 43, chief operations officer for McDonald's USA, was promoted to president of that division.

McDonald's did not give a reason for Roberts' departure, though in an email to employees Wednesday afternoon the 55-year-old said the decision was his own.

"It's with deep regret that I'm announcing my resignation, a decision that I believe is right for me, my family, my career, and the company at this time," he wrote, adding that the decision "was one of the most difficult of my life."

Roberts was named to the company's No. 2 job in November of 2004 following the resignation of former CEO Charlie Bell, who later died of cancer.

Prior to that, Roberts had served as the head of McDonald's U.S. business and has been credited with executing that unit's three-year-old turnaround under former CEO Jim Cantalupo, who died in 2004 of a heart attack.

Since 2003, McDonald's has revitalized sales at its hamburger restaurants with later hours, new menu items like higher-priced salads and chicken sandwiches, and cashless payments. The U.S. turnaround has been the most prominent, logging 40 consecutive months of positive sales at restaurants open at least 13 months, a key measure of retail strength.

In Wednesday's statement, McDonald's CEO Jim Skinner thanked Roberts for making the McDonald's system "stronger and more aligned than ever."

Skinner was named CEO at the same time Roberts was promoted to president and COO.