Redskin president Edward Bennett Williams formally announced yesterday that George Allen's contract as coach and general manager of the team has been extended through the 1981 season. Williams said Allen will get a "very substantial" increase in salary.

Williams refused to elaborate on the financial terms of the contract, and Allen was not at the morning press conference or available for comment. His mother, Loretta Allen, 91, died Wednesday, and Allen was in Albany, N.Y., for the funeral services; scheduled today.

Williams said a report in Thursday's editions of The Washington Post that Allen would be paid $250,000 a year was "inaccurate in the amount," but he declined to reveal what the coach would be paid.

Williams did acknowledge that Allen's contract was "one of the best, if not the best," in the National Football League. "I think George Allen deserves one of the best contracts in coaching, and I think he has that," Williams said.

John McKay of Tampa Bay and Chuck Fairbanks of New England have led the salary list; each is known to receive about $250,000 annually. Allen earned $125,000 a year under the terms of his old contract, scheduled to run out after the 1977 season.

Williams also emphasized that Allen's contract had been extended, no renegotiated. "We reached an agreement for the 1978, 1979, 1980 and 1981 seasons," Williams said. "He will receive his compensation in the future for those years."

Williams indicated it may still be possible for Allen to purchase stock in the club if he so desires. When Allen signed his original pack in 1971. WIlliams and owner Jack Kent Cooke gave him the option of purchasing a 5 per cent share.

Williams said that agreement was never included in the contract, only in a letter from Cooke to Allen. Allen has never exercised that option to buy, and Williams said yesterday, "If he wanted to buy the stock, I'd take it up with Jack and we'd talk it over."

Williams insisted there had never been any major problems in coming to terms with the coach.

"There never was a time where I can say in good faith there was any impasse," he said. "It would be sheer fantasy to say there was a problem over the amount of control he had or I had. It never entered into the discussions. The discussions were based on pure economics."

Williams said he spoke with Allen and the coach's attorney, Ed Hookstratten of Los Angeles, on a regular basis. One source in the organization said they had reached an agreement "several days ago."

"There was never any real pressure," Williams said. "Everybody seemed to feel because we didn't come to an agreement there was disagreement. There was no disagreement.

Allen, in a prepared statement read by his oldest son, George Allen Jr., said, "I'm pleased this is settled before training camp. It helps our entire organizations.

"A major consideration in my decision to remain with the Redskins is the ardent support that has been shown by the Redskin fans curing the past six years. The tremendous effort of our players has made these years the most gratifying of my coaching career."

"I think the timing was great," said quarterback Joe Theismann. "Everybody had to be wondering about their own future if coach Allen wasn't coming back. Even in my own contract negotiations, it was something I had to think about. I have a tremendous amount of respect for him. It's good to know he'll still be here."

"If George is happy and satisfied, then he'll take care of his soldiers," said Charles Taylor, one of the Redskin captains. "It's nice to know he'll be here, especially for the old dudes like me. I've worked with a lot of coaches, and he ranks right up there with the best. I've got two No. 1s right now."

Was he referring to Allen and Vince Lombardi? Taylor was asked, "I won't name any names, but you said it, I didn't."

In other developments yesterday, Williams put in another pitch for increased seating capacity at RFK Stadium, claiming that the team needed at least 65,000 seats to "make a profit."

He also said the club probably did not break even in 1976. The Redskins' fiscal year ended June 30 and although Williams said he had not yet seen the figures, "I suspect they weren't good."

He said he would not get involved in contract negotiations with the 12 remaining unsigned Redskins unless asked to enter the talks by his chief negotiator, Tim Temerario.

Williams added that he would like to begin discussions immediately with Metro officials in an effort to arrange for the new subway to operate on game days and nights during the preseason and regular season.