By Mary Wisniewski

CHICAGO (Reuters) - The Chicago Teachers Union voted on Thursday to allow its first strike in 25 years starting on September 10 in the nation's third-largest school district if negotiators cannot reach a contract with city officials.

The strike would start during the second week of classes for most of the system's more than 400,000 students. The last Chicago teachers' strike lasted four weeks in 1987.

The union representing more than 26,000 teachers and other professionals wants improved job security, a raise, a new curriculum and a nearly 20 percent increase in instructional time, following a push by Chicago Mayor Rahm Emanuel for a longer school day.

Public schools CEO Jean-Claude Brizard has said that with a projected $3 billion deficit over the next three years, the school system cannot afford the raise the teachers want.

The union said in a resolution posted on its website that a strike was necessary to achieve a labor contract "with acceptable wages, benefits and job protections," as well as to protest unfair labor practices committed by Chicago Public Schools.

In response, Brizard said school officials would take advantage of each of the next 11 days "and work until we reach a fair resolution for our teachers that will allow our kids to stay in school where they belong."

Brizard has said if there is a strike, the public schools would provide student services to "keep them fed and in a safe environment with positive activities."

(Editing by Cynthia Johnston and Lisa Shumaker)