Agreement has been reached on the substance of an accord that could lead to lifting of the Reagan administration's gas pipeline sanctions, and it has been sent to the White House and European capitals for approval, diplomats in Washington and Europe said yesterday.

The National Security Council was scheduled to take up the issue yesterday, an administration official said. White House spokesman Larry Speakes said he expects an announcement on an agreement "soon."

The agreement, hammered out in intensive negotiations at the State Department over the last two weeks, covers a broad outline for developing a western alliance policy on strategic aspects of East-West trade, especially on credits for the Soviet Union, sales of high technology items to the Soviets and imports of energy from Moscow, diplomats confirmed.

It also is understood that next June's economic summit meeting in Williamsburg, Va., is seen as the target date for completing the policy studies and that until then there will be a moratorium on energy purchases from Moscow, possibly on sales of certain high-technology items and contracts on the proposed second strand of the Soviet pipeline.

The administration has said from the time it imposed pipeline sanctions on companies operating in Britain, West Germany, Italy and France that it would lift them if agreement could be reached on more "effective" measures against the Soviet Union in the trade field.

"There is substantive agreement on the paper, but several procedural aspects are not determined yet. The announcement itself is a subject of controversy. Some want it longer, some want it shorter," said one diplomat, who added that at least one further meeting between European, Canadian, Japanese and U.S. diplomats is scheduled today.

One White House official, reflecting the extreme caution with which U.S. officials have approached the negotiations, said "all the pieces are not yet in place." He said he still could not say when an announcement would be made -- "today, tomorrow, next week, next month."

Just how much will be announced publicly, and by whom, is the subject of continuing negotiations among the Europeans and between major European nations and the United States, according to diplomatic sources.

This reflects the extreme sensitivity of the sanctions issue, a subject that not only has caused divisions between Europe and the United States but also within the administration.

One official, saying it was "premature" to say that full agreement had been reached, added, "I'm optimistic that when this is all over, and I can't say when that will be . . . , we will have a basis for long-term economic relations and for resolving the sanctions issue."

European and U.S. officials pointed to the painful experience of last summer's economic summit at Versailles in explaining the caution and the apparently intense negotiations over how an announcement is to be made.

Within days of a reported accord at Versailles on trade with the Soviet Union, France and the United States fell into an extremely bitter dispute about terms of the agreement. These tensions have been exacerbated many times and expanded by the sanctions dispute to include other European countries.

In this atmosphere, neither side can be pictured as backing down on the sanctions issue nor can any nation be pictured as having "given in" on negotiations for the East-West trade framework.

"The questions left open are what and how much to say," one diplomat said here. "The Europeans, or some of them, don't want the document released. They are arguing over what to say."

"This is not something the administration has cooked up to save face . . . . This is to try to put into being an expressed intent that we started down the road toward last spring" even before the Versailles summit, one European-based diplomat said.