NEW YORK (Reuters) - About 40,000 AT&T Inc workers or 15 percent of employees could walk off the job as early as Sunday after giving their union the authority to call a strike, if labor negotiations on a range of issues from healthcare costs to job security turn sour.

The Communications Workers of America said on Tuesday that workers had voted to authorize a strike in case it fails to reach an agreement with AT&T by midnight on Saturday April 7 when four labor contracts expire.

Both sides were still in talks on Tuesday in the hope of reaching agreements by Saturday. AT&T has about 256,000 employees in total.

Last August, AT&T's rival Verizon Communications had to cope with a two-week strike after contracts expired for 45,000 workers. Roughly eight months later Verizon is still negotiating with the unions for a new contract.

According to the union, AT&T is in separate talks for each of the four CWA contracts on issues such as healthcare costs, job security, benefits and work rules for employees including technicians and call center workers who handle customer inquiries.

The union said that the discussions are centered on different topics for each contract. In the AT&T West region, which covers about 18,000 employees, the union has disputes with the company on jobs and healthcare costs.

"They have a lot of concessionary proposals on the table particularly regarding healthcare costs and passing costs onto employees," said Libby Sayre, a CWA spokeswoman for the region.

AT&T declined to comment on the specifics of the talks except to say that it wants employees to "pay their fair share" of healthcare costs.

"We're really committed to working with them to reach a fair contract," said Marty Richter, a spokesman for AT&T.

Last month, AT&T reached a tentative agreement with the same union for a contract covering 9,000 workers in its wireless business.

In its wireline business alone AT&T says that it has six major contracts covering 70,000 workers that expire this year.

(Reporting by Sinead Carew; Editing by Ed Davies)