A House Judiciary subcommittee yesterday unanimously approved a 10-year extension of the 1965 Voting Rights Act, which is considered the most effective civil rights law of this century.

The panel did not resolve the key question of whether to change the provision requiring states covered by the act to get prior approval from the Justice Department for any proposed changes in their election laws. To save time, the subcommittee sent the bill on to the full committee, which is to settle that issue at mark-up sessions next week.

Sponsors want to move the bill out of committee before the August recess and through the House before the end of the year to allow time for what is expected to be a tough fight in the more conservative Senate next year. The act, which expires in August, 1982, now covers nine states and parts of 13 others.

Rep. Henry J. Hyde (III.), senior Republican on the subcommittee, felt before hearings began that those covered states, most of them in the South, had served their time in the penalty box and should be freed from the pre-clearance rule. But hearings convinced him there is still plenty of potential for voting discrimination in some states, and he now asks that there be some limited "bailout" for covered states that can meet a standard, such as 10 years without any racial discrimination in voting. Subcommittee chairman Don Edwards (D-Calif.) and full committee chairman Peter W. Rodino Jr. (D-N.J.) have been negotiating with Hyde but have not yet reached an agreement.

Liberals would like Hyde's support, hoping it would bring along some other conservative members.

Edwards, who opposes any weakening of the law, said yesterday there is "quite a lot to be said for a constructive bailout" from the standpoint of "incentive and fairness." But Edwards said he and Hyde are still a ways apart on the question of standards and which government units -- states or countries -- it would apply to.