Wichita ? An official of the second-largest union at Boeing Co.’s Wichita plant said negotiations on a new contract were being held up over a company plan that would raise medical insurance costs for many workers.

Boeing and the Society of Professional Engineering Employees in Aerospace are negotiating a contract covering 3,500 workers at Boeing’s Wichita plant.

A three-year contract that was set to expire Feb. 19 has been extended until March 19.

The company’s proposal would raise health insurance premiums to about five times their current levels for union-represented technical and professional workers who are enrolled in a traditional health plan, said Bob Brewer, the union’s Wichita director.

Premiums for a single employee would rise from $10 a month to more than $50, he said. Premiums for a family would increase from $30 a month to about $150 a month.

Medical insurance costs also would rise for employees enrolled in a Selection Plus plan, Brewer said.