Gap Inc. said Thursday that a key sales measure fell unexpectedly in June, as strong business at its Old Navy chain failed to offset sluggish sales at its namesake division and Banana Republic.

Shares of the San Francisco-based company slipped in after-hours trading following the announcement.

Gap said its revenue for stores open at least a year fell 2 percent. This is considered a key indicator of a retailer's financial performance as it strips away the impact of recently opened and closed sites. Analysts polled by Thomson Reuters anticipated a 0.7 percent increase.

The results included a 7 percent drop at its Gap stores, a 7 percent drop at Banana Republic and a 7 percent increase at Old Navy.

"Old Navy's performance was stellar this month, and we're pleased to see the brand continue its strong quarter-to-date momentum," Glenn Murphy, chairman and CEO of Gap Inc., said in a statement. "Despite softer June results at Gap and Banana Republic, we remain focused on delivering in the upcoming fall season."

The company reported total revenue of $1.54 billion for the five-week period that ended July 5.

Gap shares fell 62 cents to $40.34 in after-market trading after closing down 45 cents, to $40.97 in regular trading.