The Supreme Court yesterday agreed to consider the Reagan administration's argument that companies should not be allowed to use the federal antitrust laws just to protect themselves from increased competition.

The decision came in a case involving a suit by a Colorado beef-packing company, which is seeking to stop the planned merger of two of its competitors.

The Justice Department views the case as an important part of its current plans to press for reform of the federal antitrust laws, which prohibit corporations from restraining competition.

In a friend-of-the-court brief, the department joined Cargill Inc. in asking the court to allow Cargill's acquisition of Spencer Corp., the nation's third-largest beef packer.

Justice said that the antitrust laws are too often used by weak companies to thwart mergers that would create tougher competition. "The antitrust laws would be perverted if they could be used to thwart transactions that would enhance competition," the government wrote.

In this case, Monfort Inc., the nation's fifth-largest beef packer, sued successfully in a lower court to stop the merger of two of its rivals, Excel Corp., a Cargill subsidiary and the second-largest beef packer, and Spencer Corp. Monfort had argued that the combination would violate antitrust laws by creating a corporation so powerful that it would lessen competition in the beef-packing industry.

The proposed merger "was found to be unlawful by the District Court of Colorado and by the Court of Appeals for the 10th Circuit ," said James E. Hartley, an attorney for Monfort.

Cargill argued that Monfort was merely afraid of increased competition. Robert Hanley, an attorney for Cargill, said the antitrust laws "were designed to protect competition, not competitors."

The government, however, did not ask the Supreme Court to overturn the lower courts' conclusions on whether Excel and Spencer violated the antitrust laws. Rather, it asked the high court to rule that Monfort should not be allowed to sue -- that it lacked standing -- because it had not suffered, and was not likely to suffer actual antitrust injury from the proposed merger.

The government used a similar argument when Chrysler Corp. sued on antitrust grounds to block a joint venture between Toyota and General Motors Corp. The Justice Department got the suit tossed out when it argued that the joint venture would enhance competition, and that Chrysler should not be allowed to sue.

The Justice Department views this as one of the most important antitrust cases this year, and believes a ruling in its favor would dovetail with White House plans to propose reforms of antitrust law.

Commerce Secretary Malcolm Baldrige has urged repeal of the law used by Monfort -- Section 7 of the Clayton Act, a pillar of antitrust law -- which prohibits mergers that "may" lessen competition or "tend to" create a monopoly. Baldrige charges that the law discourages companies from attempting mergers that would create stronger firms at a time of decreasing U.S. competitiveness.

President Reagan's chief economic advisers have recommended that the White House propose legislation to revise the Clayton Act to require the type of economic analysis that they think appropriate in evaluating proposed mergers. The proposal, which is likely to meet some resistance in Congress, would have required the lower courts to evaluate the Monfort case differently, one administration official said.

The case presents an unusual opportunity for the high court to address these issues, the government wrote. Few such cases reach the Supreme Court because most companies enter into merger agreements that allow them to abandon the transaction if an antitrust suit is filed.