Metro has delayed completion of the inner-city Green Line and used funds inefficiently by working on many rail segments at once, the General Accounting Office has concluded after a study of the construction program.

When work begins on the final 26 miles in the planned 101-mile system, the GAO said, funds should be concentrated first on finishing Green and Yellow Line segments that will serve densely populated areas before suburban track that is expected to attract fewer riders.

Metro's current "piecemeal" approach has up to six sections of line under construction at once, leading to a slow pace on important lines, the GAO said in a report released yesterday.

The recommended changes would "make the most cost-effective use of limited federal funds," the report said, and let the public "begin using Metrorail as soon as possible."

Metro disputes the GAO's conclusions, saying that it is spending money as fast as is practical on some projects, such as the Orange Line to Vienna, scheduled to open in 1986.

Starting work on many lines as permits become available allows Metro to counter inflation, officials said.

"We believe in scheduling money in the most cost-effective manner," said General Manager Richard S. Page.

He said that delays in Green Line construction grow out of problems in obtaining agreement on its route and other permits.

Adopting GAO's changes also could damage Metro's political situation, the transit agency said in a response submitted to the GAO findings, because the current system gives each of the eight local local governments in Metro something to show for its taxpayers' money.

The GAO proposal could be moot, since the Reagan administration has said it currently will fund only a 75-mile system, with money for the final 26 miles depending upon improvements in the economy.

The GAO report also called for the federal government to place a ceiling on the share of a given contract that it will pay, to give Metro added incentive to control costs, and recommended that Metro periodically be required to review its funding sources for operating subsidies.