A previously unnoticed mandate of the D.C. home rule charter could require the City Council to cut $17 million from the 1979 operating budget proposed by Major Walter E. Washington, The Washington Post learned yesterday.

The reduction in the $1.3 million operating budget would be neccessary because the $317 million federal payment assumed by the mayor is larger than the $300 million payment that exceeds the currently authorized level. There is legislation pending in Congress to raise that ceiling, but it is not expected to be approved before Dec. 15. That is when the Council expects to take final action on the budget for the fiscal year beginning Oct. 1, 1978.

Comer S. Coppie, the city's budget director, said that approval by the Council of a budget that included a U.S. payment of more than $300 million "certainly raises legal questions; whether or not its illegal is a judgement."

Coppie said the problem, which he termed more technical than serious, would require the city to reduce its originally submitted budget expenditures by $17 million and to seek restoration of those funds later. That request would have to come at some point after legislation increasing the federal payment is approved in Congress, he said.

Coppie could not say yesterday what programs might be curtailed in order to make way for the $17 million reduction. Major Washington, interviewed briefly as he left the meeting, said that they office would recommend spending reductions to the Council within the next several days.

Yesterday's disclosure was the subject of an hour-long closed door afternoon meeting in the office of City Council Chairman Sterling Tucker, that incuded the major, some of the city's top legal officials several members of the Council. Tucker refused to discuss details of the meeting.

Several Council members already were looking skeptically at the unprecendented $317 million figure and calling it a "soft" forecast. In addition, the city is expected to face steep uphill fight, in Congress for such an appropriation because key congressional leaders have serious doubts about the efficiency of D.C. city government.

The payment is the federal government's annual allocation to the city to compensate for revenues that the District loses because of tax exemption of federal property in the city and the additional municipal costs incurred from being the nation's capital.

Although the current authorization, as set in the home rule charter, is $300 million, the city never has received that amount. A House-Senate conference committee still is trying to resolve the 1978 payment level. The House has recommended $295 million while the Senate has recommended $276 million. The city had requested the full $300 million.

Coppie said yesterday that his inclusion in the major's budget of a $317 million figure was based on a recommendation by the White House that it would support that figures as a new authorization level and was the proper procedure to follow.