By Daina Beth Solomon

MEXICO CITY (Reuters) -Mexico's antitrust watchdog, Cofece, said it has summoned a supermarket retailer to respond to findings of "probable responsibility" for competition law violation, and Walmart's Mexico unit announced it was facing such a probe.

A source familiar with the matter said the company in question was Walmart's Mexico unit, which on Friday reported it would face an antitrust panel into "alleged relative monopolistic practices" after a three-year Cofece investigation.

Asked if Cofece's Monday statement referenced the same matter, Walmart de Mexico, often known simply as Walmex, said it had received a notification from the authority on Friday. Walmart owns 71% of Walmex.

"The company is reviewing the notification, with 45 business days to present arguments and evidence," it said, adding that it was confident of having acted lawfully to ensure good prices and product supply.

The notification was related to the supply, wholesale distribution and marketing of consumer goods, Walmex said.

Walmex led losses on Mexico's S&P/BMV IPC stock index on Monday, with shares down 5.5%.

Analysts have warned that shares could see some short-term volatility due to uncertainty over the potential impact on operations, which the company said it could not yet estimate.

The company in question will now have an opportunity to defend itself in a trial-like procedure against findings of "probable vertical price fixing and/or other relative monopolistic practices," Cofece said in a statement.

Such violations can carry a fine of up to 8% of a company's annual income in Mexico.

Cofece said the food and beverage sectors are priority areas to monitor for anti-competitive behavior, noting that Mexican families spend more than half their income on consumer goods.

Walmex is the biggest supermarket chain in the country, with 2,890 locations.

(Reporting by Daina Beth Solomon, additional reporting by Noe Torres; Editing by Brendan O'Boyle, Stephen Eisenhammer and Cynthia Osterman)