Oct 14 (Reuters) - A Missouri judge on Wednesday struck down a planned increase in the city of St. Louis' minimum wage that was challenged by a state chamber of commerce, retail and restaurant associations, and other business groups.

Mayor Francis Slay said he planned to appeal the ruling by St. Louis Circuit Court Judge Steven Ohmer. The minimum wage would have increased to $11 by 2018 in steps starting on Thursday. Missouri's minimum wage is $7.65.

"We will appeal the ruling with the hope that higher courts will affirm the city's authority to adopt its own minimum wage," Slay said in a statement.

The judge ruled that the minimum wage increase conflicted with state law.

The Associated Industries of Missouri, a business trade group and plaintiff in the lawsuit with other business groups, had argued that the increase violated state law and a wage increase could force private employers to let go workers.

"No local government has ever had the authority to issue an ordinance that directly violates state law. We believe the court made a correct decision," group President Ray McCarty said in a statement.

Nationally and in Missouri protests led by fast-food workers have added pressure to raise minimum wages, prompting some local governments to act.

(Reporting by David Bailey in Minneapolis; Editing by Eric Walsh)