Based on Monday's closing price, the transaction would be worth about 1.28 billion euros ($1.76 billion). Two sources close to the deal said the stake was bought on the market.

A source close to Carrefour referred to the acquisition as "a friendly approach".

Motier - the holding company of the Moulin family which owns 100 percent of Galeries Lafayette - has acquired 44.2 million shares of Carrefour, or 6.1 percent of its capital, Motier said in a statement.

"This is a long-term strategic investment and reflects our confidence in the growth potential of Carrefour," Motier said.

It added, however, that the group would continue to focus on its core department store business and would invest the necessary funds in that business in the years ahead.

"The Moulin family were the founders of supermarket chain Monoprix, so in a way the Carrefour investment is a return to their first business, which was supermarkets," a source close to Motier told Reuters.

With the acquisition, Motier becomes Carrefour's second-biggest shareholder after the consortium of Groupe Arnault and private equity group Colony Capital, which together own 14.77 percent of Carrefours' capital and 20.06 percent of the voting rights, according to Carrefour's annual report.

Arnault and Colony own 8.88 and 5.89 percent respectively, Thomson Reuters data shows.

Norges Bank Investment Management (NBIM), which manages Norway's sovereign wealth fund, held 2.19 percent of Carrefour stock at the end of 2013.

($1 = 0.7277 Euros)

(Reporting by Geert De Clercq, Dominique Vidalon and Matthias Blamont; Editing by Mark Potter and David Evans)