CHICAGO - The Chicago Teachers Union said Monday that nearly 90 percent of teachers voted in favor of giving the union authorization to strike if a deal on a contract with the city cannot be reached this summer.

Union leaders said the vote, which began last week, gives teachers the legal authority to call a strike in the fall and gives them added leverage at the bargaining table.

State law requires 75 percent approval.

The union president, Karen Lewis, called the result “an indictment of the state of the relationship between the management’’ of the Chicago public schools and the teachers.

If a strike is called for the next school year, it would be the first school strike in Chicago since 1987, when teachers stayed off the job for 19 days.

The head of the Chicago public schools, Jean-Claude Brizard, said he is disappointed that the union took a strike authorization vote before an independent fact-finder presented a recommendation.

The teachers’ frustration largely centers around Mayor Rahm Emanuel, who rescinded a 4 percent raise last year and then began pushing for a longer school day.

Teachers say the mayor - and now the district - have not offered them enough money to make up for the added time.