A U.S. plan to convert more than 60 oil and gas-fired utilities to coal would boost air pollution over the Northeast by more than 25 percent and sharply increase so-called "acid-rain" showers falling in Canada, according to a Department of Energy memorandum.

The plan, outlined in the memo late last month to James T. McIntyre, head of the Office of Management and Budget, has been a source of particular concern to Canadian officials. The Canadians were recently assured by senior U.S. enviormental officials that coal conversion under President Carter's energy program would not increase air pollution.

Acid rain -- which is usually caused when coal-fired power plant emissions and automobile exhaust combine with atmospheric moisture -- is an unusually sensitive subject with the Canadians. A Canadian government study has estimated that acid rain may kill fish in 48,000 lakes north of the border over the next 20 years.

According to the Canadian study, the United States sends about 25 million tons of sulfur dioxide pollution into Canada each year from power plant emissions and other sources. The Canadians send back only about 5 million tons each year. Officials on both sides of the border are negotiating over lowering the emission levels that would be allowed.

George Rejhon, an enviromental counsellor in the Canadian embassy here, said yesterday that Canada is studying the information in the Energy Department memo.

"If his information is substantially correct," Rejhon said, "we would obviously be very concerned. It would indicate a large increase in pollution emissions at a time when we are negotiating with the U.S. to lower them."

Rejhon said that about three weeks ago Douglas M. Costle, head of the Enviromental Protection Agency met with top Canadian enviromental officials. Costle assured the Canadians at the meeting that sulfur dioxide emissions in the Northeast and Midwest were decreasing and would continue to do so, Rejhon said.

Costle could not be reached for comment yesterday. David Hawkins, assistant EPA administrator for air programs, acknowledged that there had been no mention of the increased pollution under the proposed conversion plan during the meeting with the Canadians.

"We didn't calculate the figures on coal conversion until after the meeting," explained Hawkins, who attended the meeting.

Hawkins said there was no plan to notify Canadian officials of potential pollution increases unless the United States formally adopts the coal conversion proposal.

The coal conversion plan outlined in the Jan. 22 memo to McIntyre is the second attempt by the Carter administration to cut in half the oil consumption by electric utilities. A copy of the memo was made available to The Washington Post.

The Administration is seeking to amend the federal Power Plant and Industrial Fuel Use Act to speed utility conversions to coal and cut daily oil consumption by up to 550,000 barrels by 1985.

The original version of the plan was killed after opposition from utilities and state officials. Opponents said it was too ambitious, too costly and would be hampered by existing enviromental standards.

The new version outlined in the Energy Department memo suggests an immediate conversion of 62 power plants to coal, most of them in the Northeast. According to the memo, the conversions generally would be exempt from federal enviromental regulations. Instead, they would be subject to less stringent state air quality plans.

Under state regulation, the conversions would add almost 600,000 tons of sulfur dioxide pollution to the atmosphere nationally and 230,000 tons of sulfur dioxide over the Northeast.

The memo said that the latest version of the coal conversion plan has won support from utilities and congressional leaders, but is opposed by the EPA.

The enviromental agency criticized the Energy Department for failing to consult it on the plan until Jan. 21, the memo said.

When the agency was consulted, it warned that the new plan would expose such densely populated areas as New York City to increased fallout of radiomclides, a pollutant recently classified as hazardous by the environmental agency, the memo notes.

The memo also said EPA had recommended that power plants converting to coal be required to meet existing federal clean air standards. The cost of meeting tougher federal standards would be an additional $1 billion and would mean no additional pollutants in the atmosphere, the memo pointed out.