In a memorandum to its player representatives, the National Football League Players Association has accused the general counsel to the NFL Management Council of sabotaging $1.55 million in Labor Department grants to the NFLPA by alleging falsely that the money was being used for union activities.

Cancellation of those grants, which were intended to support summer youth camps, will cost the union $80,000 in staff and overhead expenses it had hoped to cover with the government money, according to the July 28 memorandum, which is signed by NFLPA President Gene Upshaw and Treasurer Carl Barisich.

A copy of the memorandum was obtained by The Washington Post. It contends a letter written to the Labor Department by Sargent Karch, general counsel to the Management Council, caused the cancellation of government grants.

Because of the cancellations and a reduction of corporate promotional payments to the NFLPA, the union says it is having "a cash flow problem" and faces a possible deficit this year of $183,000.

The memorandum also says the NFL pressured Shell Oil and Coca-Cola into dropping promotion campaigns with the NFLPA, costing the union another $100,000.

Spokesmen for Shell and Coca-Cola confirmed the nonrenewal of the promotional campaigns, but said they were business decisions and there had been no pressure from the NFL. Jim Miller, a spokesman for the Management Council, the league's labor-negotiating arm, called the accusation "another ridiculous union charge."

Karch confirmed he had written the Labor Department in January after examining the NFLPA's annual financial report, which it is required to file annually and which is a matter of public record.

In the letter, Karch said he was "concerned about the possibility that the NFLPA may have used government funds to seek to create an economic advantage in collective bargaining . . . the NFLPA may have used CETA funds to fully or partially compensate employes engaged in union activity, as well as for other improper uses promoting unionization."

Karch said he was alleging no specific improprieties, only raising questions. In particular, he questioned $40,225 in salary paid to Brig Owens, then the NFLPA's director of community services, from government grants in the year ending Nov. 30, 1980. He also questioned $475,000 in unaccounted government funds in the NFLPA's statement and the payment of $881,857 in government money in stipends of less than $10,000 each to an unspecified number of people.

But Ed Garvey, executive director of the NFLPA, said Owens at that time was putting in a 40-hour week on the government-funded programs, which consisted chiefly of the operation of five summer camps for approximately 6,000 children. Moreover, he said, the union paid one-third of Owens' salary. Any government money unaccounted for would have been returned or carried over into the next year's program, he said, and the stipends of less than $10,000 represented payments to camp counselors, some of whom were NFL athletes.

In April, the NFLPA was notified that the Labor Department grants -- $350,000 from the Job Corps program and $1.2 million from the Youth Development Program -- were being canceled, and the union decided not to continue the camps. A spokesman in the department's public affairs office said the cancellation was part of a general cutback in such programs and was not related to Karch's letter. He said Karch's questions had been investigated and no improprieties had been found.

Had the summer camps been funded this year, it would have been their fourth year of operation. Up to 500 NFL players have participated in the program, in which youngsters are organized into simulated community situations where they hold elections, publish newspapers and face other tasks and issues they might meet in real life.

"We were trying to let all the kids be able to dream by giving them some idea of how to make the dream become a reality," said Owens.

The corporate promotional campaigns involved use of posters of NFL players by local Shell service stations and pictures of NFL players on the inside of Coca-Cola bottle caps. Spokesmen for both companies said the campaigns were routinely phased out as the companies moved to other promotional tactics.

Garvey said the loss of the government money hurts because the NFLPA had been counting on it. "But it won't affect our position at the bargaining table," he said.

The NFLPA and the NFL Management Council are attempting to negotiate a new contract to cover wages and working conditions, but so far have made only minimal progress. The old contract expired July 15, and since then the league has revoked its automatic union dues checkoff.

Upshaw and Barisich asked the player representatives to begin collecting dues at the rate of $25 a week for the next 20 weeks to meet the members' dues obligations for the coming year. NFLPA dues are $850 this year, but $350 was prepaid last year. Mediator Offers Help Associated Press

The head of the Federal Mediation and Conciliation Service said yesterday the agency would try to help settle the stalemate between the National Football League and the players union if asked to do so.

Kay McMurray, former chairman of the National Mediation Board, was recently confirmed by the Senate as the director of the mediation board. He replaces Kenneth Moffett, who was instrumental in ending last year's baseball strike.

The mediation board can become involved in a dispute by the invitation of one or both parties or by its own intervention. Jack Donlan, executive director of the NFL Management Council, the league's bargaining arm, and the union's executive director, Ed Garvey, say there are no plans to seek federal mediation.