Paul Marchant, the chief executive of Primark, has resigned “with immediate effect” from his role leading the discount clothing retailer after an allegation about his behavior, the company announced Monday.

Mr. Marchant, who joined Primark in 2009, left after the company hired lawyers to investigate “an allegation made by an individual about his behavior towards her in a social environment,” Associated British Foods, Primark’s parent company, said in a statement.

It said that he cooperated with the inquiry, “acknowledged his error of judgment and accepts that his actions fell below the standards expected by the company.” Mr. Marchant apologized to the individual, the board and others at Primark, according to the statement.

“I am immensely disappointed,” George Weston, the chief executive of Associated British Foods, said in the statement. “We believe that high standards of integrity are essential,” he added.

Primark is a low-cost clothing giant that has become synonymous with a certain type of cheap and expendable fashion. Arthur Ryan, its founder, fought to keep prices down.

Mr. Ryan appointed Mr. Marchant, who had long worked in fashion retail, to be his successor as chief executive. Under Mr. Marchant, Primark flourished.

It has more than 450 stores in 17 countries and employs some 82,000 people. The retailer recorded sales of 9.4 billion pounds ($12.2 billion) for its latest financial year, an increase of 5 percent from the year before.

The retailer, which was founded in Dublin in 1969 and expanded to England in 1973, is now a staple on main streets of cities in the region: Stores in Britain and Ireland account for about half of its sales.

Primark has been making inroads in the United States, which accounts for about 5 percent of its sales. As of January, the company ran 29 stores in the country, and signed 17 leases for future locations.

“Under Paul’s leadership,” the retailer said in an online biography of its executives, “Primark has become a truly international business.” The company noted that Mr. Marchant led an expansion into Europe and the United States, “more than doubling our store footprint since joining.”

The share price of Primark’s parent company fell about 4 percent after the announcement, near its 52-week low.

Primark did not immediately respond to a request for comment.