Two unions representing a total of about 2,000 workers at the Washington Gas Light Co. have voted to reject contracts offered by the company and have authorized their members to go out on strike if contract talks fail.

The separate contracts for both unions, which represent about 1,400 maintenance and mechanical workers and about 600 clerical workers, expired May 31, but all sides had agreed to extend them until at least midnight yesterday.

The International Union of Gas Workers, which represents the maintenance and mechanical workers, Monday voted down the two-year contract offered by the company, at the same time authorizing a strike. Yesterday that union agreed to extend its contract through midnight Thursday to provide time for further negotiations.

The clerical workers' union voted yesterday to reject its proposed contract and authorize a strike.

Michael Cowan, chief negotiator for the Office & Professional Employees International Union, Local No. 2, which represents the clerical workers, said last night his union has agreed to a similar extension.

A federal mediator has been meeting with company and union officials since last week.

"The most important thing is to keep our people working, and we have a concern for the customers out there also," said Robert Pellegrino, a member of the Gas Workers negotiating committee.

Washington Gas has about 560,000 customers in Washington and throughout the Maryland and Virginia suburbs.

Paul Young, a company spokesman, said that management and supervisory personnel are prepared to operate the company in the event of a strike and that the company is confident it will be able to provide essential and emergency services.

Washington Gas, which has been in business in the District since 1848, has had only one previous strike, which occurred in 1961 when 1,300 mechanical and maintenance workers, who were then represented by a different union, conducted a 15-day walkout, according to Young.

In the current labor dispute, Young said, the company has offered each union a two-year contract that calls for a 6 percent pay increase for each year and a slightly more than one percent increase in fringe benefits spread over the two years.

Donald J. Heim, chairman and president of Washington Gas, said in a statement that the company's offer is "completely equal" to contracts recently signed by other utility companies and "balances the interests of employes, rate-payers and stockholders."

Pellegrino said research by the gas workers union shows that Washington Gas' pay scale is below salaries paid by other major utility companies.

While expressing dissatisfaction with the company's pay proposal, officials of both unions said that the dispute centers on demands for improved fringe benefits, particularly for health insurance and pensions.