HOUSTON, Sept 14 (Reuters) - BP Plc plans to cut staff, both hourly and salaried, at its Texas City, Texas, chemical plant as it reduces production of paraxylene, which is used to make polyesters, according to sources familiar with the company's plans.

The layoffs will include 24 hourly and 12 salaried employees, equal to 15 percent of the plant's work force, one of the sources said.

A BP spokesman said the plant is shutting a paraxylene unit that has been idle for over a year due to market conditions and to make better use of resources.

Two other paraxylene units will remain open at the plant, which has annual production of 1.4 million tons, according to BP.

One of the sources said the United Steelworkers union, which represents hourly workers at the plant, was challenging the layoffs of its members.

(Reporting by Erwin Seba; Editing by Lisa Shumaker)