Agriculture Secretary John R. Block announced yesterday that U.S. and Soviet officials will meet next month in London to discuss possible new sales of American grain to the Soviets.

The announcement came six weeks after President Reagan lifted the embargo on grain sales to the Soviet Union.

The meeting, scheduled for June 8-9, will be the first official grain-sale discussion between the two countries since President Carter imposed the embargo in January, 1980, following the Soviet invasion of Afghanistan.

The stated purpose of the meeting is to discuss further sales under the current five-year Soviet-American grain agreement, which expires Sept. 30. But it could also mark the first tentative step toward reopening negotiations aimed at reaching a new long-term agreement to replace the one that is now in place.

Whether such negotiations could be set in motion during the London meeting, however, remains unclear. David Lane, an Agriculture Department spokesman, said the United States has already informed the Soviets of its willingness to listen to proposals for a new long-term agreement. But Lane said the next move is now up to the Soviets and that the U.S. delegation to the London talks will not raise the subject of a new long-term pact.

The Soviets, according to various sources, have at least until recently said they are not interested in a new agreement with the United States.

The Soviets, however, can buy additional grain under the existing agreement before it expires and have indicated they are interested in doing so. a

Under the agreement, the Soviets can purchase up to 8 million metric tons of American grain a year without receiving any special government approval. For amounts exceeding 8 million metric tons, government approval is necessary.

Lane said the Soviets have purchased close to 8 million tons already this year and in London will indicate how much more they are seeking. Lane said the Agriculture Department has no idea how much this may be.

In 1976-77, the first year of the agreement, the Soviets purchased 6.1 million metric tons, just over the minimum 6 million metric tons they are required to buy every year under the agreement. During the next three years, Soviet purchases were 14.6 million and 15.3 million metric tons.

For 1979-80, the year of the embargo, the Soviets had received permission to purchase 25 million metric tons of grain from American firms before the invasion of Afganistan. As a result of the embargo, they were allowed to buy only the 8 million metric tons for whichl they required no additional government approval that year.

Block announced the upcoming grain negotiations in London, where he was meeting with British agriculture officials as part of a two-week European tour. He said that, in addition to new grain sales to the Soviets and other needs, they may have, the talks will include a "review of the world crop situation, crops in the United States and crops in the Soviet Union."

Despite Reagan's hard-line stance against the Soviet Union, the administration has made it clear it would consider a new long-term grain agreement with the Soviets provided there are no new episodes such as the Afghan invasion. Block has publicly expressed this.

Lane noted that because of the embargo experience, negotiations over a new long-term agreement are likely to be protracted. He also said that the Soviets now have new long-term grain sale agreements with Argentina and Canada and thus may not feel any urgency in attempting to negotiate a new agreement with the United States.