Acting under orders from Congress, the Department of Agriculture yesterday announced plans for stiff new duties on imported sugar that could raise supermarket prices of the commodity by 3 1/2 cents a pound.

Administration officials estimated that the new regulations will cost consumers from $400 million to $800 million a year in higher prices of sugar and sweeteners.

The national average retail price for a pound of sugar is 21.68 cents a pound - or $1.08 for a five-pound bag.

Under Secretary of Agriculture John White also acknowledged at a press conference that the new duty rates will hurt developing countries that export sugar and will probably produce windfall profits for big corporations and importers which have been accumulating unusually large stocks in anticipation of the action.

The new duties are seen as certain to complicate the Carter administration's international trade problems. The administration already has encountered protectionist sentiment in steel, shoes, television and several other industries. Officials fear the sugar action could encourage protectionists to press for more victories.

Even before the sugar decision was announced, European officials told the administration that they objected to it as unlateral interference with free trade.

White said that President Carter's approval of a new tariff and fee schedule is expected within a day.

Administration officials have been notably unenthusiastic about the plan, but say it is necessary to bring wholesale domestic sugar prices up to the floor of 1 1/2 cents a pound established by Congress in this year's farm bill.

The administration had wanted a system of subsidies to farmers which would have compensated growers for slumping across the board. But it lost out to congressional interests advocating a price support program. The congressional plan was supported by among others, the multimillion-dollar corn sweetener industry, which has trouble competing in the lucrative soft drink and ice cream market when the rival sugar industry is offering its sweetners at low prices.

Under the new duty schedule, the tariff is to be raised from 1.87 cents to 2.81 cents a pound, and additional fee of up to 3.3 cent will be imposed.

With world sugar prices now running slightly more than 7 cents a pound, the tariffs and fees of up to 6.1 cents should bring the price up to the floor level when shipping costs are added.

If the price does not reach the desired level, the administration could announce a sharp reduction in its quota of imported sugar, now set at 7 million tons annually. However, officials said they hope to avoid this step.

Imports - which provide about half the country's sugar needs - are now running at about 5 million tons.

White sought to take some of the sting out of the tariff action by pointing to the "emergency" prevailing in the domestic sugar economy, where the prevailing price is now below the cost of production. "The question is whether we want to have a domestic sugar industry," he said.

One positive factor in the new plan is that it should net the U.S. Treasury between $360 million and $550 million in new duties, officials said. But that plus must be set against the minus of increased costs to consumers.

Big importers and users, anticipating the government's action, have built up enormous stockpiles - totaling between 1 million and 1.5 million tons - in expectation of higher prices. Also, some 400,000 tons of sugar is reported to be on the high seas heading for the United States.White said this sugar would not be subject to the higher duties. Asked if this was an unusual quantity to be under way at one time, an official replied: "you bet it is."

Officials said that developing countries - inevuding a number of poor ones that depend heavily on sugar exports for the bulk of their foreign revenues - will be the losers, along with American shoppers.

The higher prices are expected to reduce American imports by an uncertain amount, the officials said, and this tend to depress world sugar prices still further.

Also, 17 small countries that export sugar duty free to the United States under a special concessional arrangement will lose their favored treatment under the new regulations.

Until two years ago, sugar was imported into the United States under a complex system of nation-by-nation quotas that often was subject to behind-the-scenes lobbying. Since then, the sugar trade has been free, except for the small tariff that prevailed until yesterday.