The Reagan administration agreed yesterday for the first time to support new trade negotiating authority that includes as a goal assuring American companies "market access" equal to that given foreign companies in the U.S. market.

In outlining the policy to the Senate Finance subcommittee on trade, U.S. Trade Representative William E. Brock made clear that the administration would reject so-called "reciprocity" language that would require the United States to retaliate automatically against countries that establish difficult barriers to U.S. exports.

Instead, the administration will pursue market access "as a negotiating" objective, and always "within the framework of GATT"--the 87-nation General Agreement on Tariffs and Trade--Brock assured Sen. Bill Bradley (D-N.J.).

Brock said the administration will not endorse reciprocity language that would mandate a retaliatory step by the president against either a specific country, or to force equality in a particular trade sector or product.

"Any bill that would mandate action on that sole ground would be a severe handicap. I would worry about creating this new kind of action," Brock said.

He later told reporters that the administration will seek "omnibus" language that would draw from the "positive approach" of scores of bills that have been introduced in Congress.

Subcommittee Chairman John C. Danforth (R-Mo.) promised Brock that every effort will be made to avoid "a Christmas tree bill"--legislation that incorporates a host of pet congressional projects, in this case to protect declining industries by one gimmick or another.

Many U.S. trading partners, as well as free-trade advocates here, have feared that reciprocity as broadly advocated by critics of Japan on Capitol Hill would be a back-door route to outright protectionism, which not only would be counterproductive, but would violate the basic U.S. commitment to the GATT and to multilateral trade principles.

But Brock's announcement was at least a partial victory for advocates of reciprocity who had been insisting that Japan and some European nations had devised subtle nontariff barriers that are working unfairly against U.S. companies, and must be countered in some way.

Brock acknowledged that the goal of free trade isn't served "by ignoring attacks on it by others or by not pursuing increased market access for our goods, services and investment." But he reiterated that new legislation "must be absolutely consistent with current obligations under the GATT and other international agreements."

Brock also indicated that an administration-backed bill would include enforcement authority, similar to Section 301 of existing law relating to goods, to cover exports of services, "an area where we are experiencing expanded trade opportunities and growing barriers to them."

The trade representative also wants new authority for the president to modify the tariff structure governing high-technology items, which he said "would assist our efforts to obtain increased market access for U.S. goods." He didn't mention Japan--but part of the administration's current dialogue with Japan relates to getting American high-tech items into what U.S. companies charge is a protected Japanese market.

Commerce Secretary Malcolm Baldrige, who reportedly had been more of a hawk than Brock on the reciprocity issue, yesterday supported Brock's position. But he pointed out that various nontariff barriers "have considerably weakened adherence to free-trade principles and made more sharply unfair the denial of market access on an equal basis."

Danforth, a strong backer of reciprocity legislation, expressed his gratification with the administration's position and said that, on the basis of what he had heard, compromise language could be worked out "in less than an hour." Finance Committee Chairman Robert Dole said that, although priority must be given to the president's budget, an effort will be made now to work out a compromise between the various reciprocity bills and the administration's more restrained approach "within three to four weeks."

The thrust for new legislation comes largely from the swelling Japanese trade surpluses at a time of national recession here resulting in growing unemployment. Although Japan has taken a series of significant steps to liberalize trade barriers, U.S. officials are pressing Japan to do more. Sen. Lloyd Bentsen (D-Tex.) also listed a wide variety of complaints against the European Economic Community and Canada.