The Reagan administration announced yesterday that it will relax a 10-year-old federal bumper standard which required auto manufacturers to make front and rear bumpers that would protect an automobile in low-impact crashes up to 5 miles an hour.

Raymond A. Peck, the administrator of the National Highway Traffic Safety Administration, ordered the current standard be rolled back to 2 1/2 mph, after concluding that the modification would save consumers an estimated $300 million a year.

What's more, Peck said the NHTSA may allow a further relaxation in the new standard so long as there is a net benefit to consumers.

Peck's decision drew quick praise from the auto industry, which had made the relaxation of the 5-mph standard one of the priorities in its campaign to reduce costs.

Consumer groups and insurance industry officials quickly denounced Peck's decision, however, charging that it would result in less safety and sharply higher insurance rates, which would far outweigh any savings consumers gained in the price of a car or in fuel economy.

Peck contended that the change to a 2 1/2-mph standard would result in a net savings of $28 over the life of a car. According to the NHTSA's statistics, the agency expected a $65 increase in insurance rates and higher repair costs over the life of the car. But Peck said this jump would be more than offset by the $93 savings gained through lower sticker prices and fuel savings.

"Our evidence is very clear that, if we do get a weaker bumper as a result of this--and it is inevitable that some cars will be made with them--the amounts of damage will increase dramatically and this in turn will translate into higher insurance costs for owners of these cars," stated Brian O'Neill, vice president for research at the Insurance Institute for Highway Safety.

O'Neill said he expects an increase of between 15 and 20 percent in collision costs as a result of Peck's decision. Under that assumption, between $35 and $40 would be lost in first year, he said.Two and a half miles an hour is walking speed--that's the level of protection that we'll be getting," said consumer activist Ralph Nader.

Peck, however, contended that all his calculations were based on insurance industry figures given to the NHSTA.

The auto industry agreed with Peck's decision. "GM is encouraged by the new evidence of the Reagan adminitration's commitment to make automobile regulations as cost effective as possible," said a General Motors Corp. spokesman.

The auto industry received earlier evidence last fall when Peck repeales the NHTSA's requirement that all 1984 cars be equipped with airbags or automatically closing seat belts-a rule the industry has fought from the day it was issued.

"We are confident that the 2 1/2 mph standard is overall a better value for or coustomers," GM said. CAPTION: Picture, The administration says that rolling back the current standard for bumpers will save consumers about $300 million a year. By Craig Herndon -- The Washington Post