President Reagan plans to lift the grain embargo against the Soviet Union today, after the stock and commodity markets have closed for the weekend, according to White House officials.

The embargo has troubled Reagan since his inauguration. He campaigned against it and pledged to lift it, but when he took office he was persuaded that he would send the Soviet Union a wrong signal if he followed through on his pledge at that time.

White House counselor Edwin Meese III, who insisted to a group of newspaper editors yesterday that "I have no basis to tell you when [Reagan] might make a decision," added a new note. He said that one factor the president would take into account in making an embargo decision would be whether he has been in office long enough for the Soviet Union to have come to understand the signals he is sending them.

The embargo, imposed by President Carter in January 1980 in an attempt to punish the Soviets for their invasion and occupation of Afghanistan, apparently is being lifted with the justification that the Soviets have not invaded Poland or sponsored an internal crackdown against its labor unions.

Reagan has consistently opposed the embargo on the grounds that it is unfair to farmers, demanding that they shoulder a burden he says should be borne more generally.

Farm groups have pressed him to honor his campaign pledge and the president repeatedly has said he would like to lift the embargo.

Secretary of State Alexander M. Haig Jr. persuaded the president early in the administration, however, that with the Soviet threat to Poland increasing, it would be inappropriate, and inconsistent with the hard-line stand against Moscow that the Reagan administration was seeking to establish, to lift the embargo.

Haig, Meese and Agriculture Secretary John R. Block, a strong advocate of ending the embargo, met at the White House Tuesday to discuss the action.

Haig and other administration officials have said recently that the Soviet threat against Poland appears to have diminished since tensions reached new highs during Soviet maneuvers at Poland's borders early this month. Haig still thinks it is a mistake to lift the embargo, however, State Department sources said.

White House deputy press secretary Larry Speakes insisted yesterday that Reagan "has not yet made a decision on whether to lift the embargo." Speakes reminded reporters that Meese also said yesterday that Reagan has not yet decided.

CBS News reported Wednesday night that an announcement of a phase-out of the embargo would be made today. Meese, speaking yesterday to a meeting of the American Society of Newspaper Editors, appeared to deny that it will be a phased lifting.

"I don't know how to phase out an embargo either," Meese told a questioner.

He said he gave little credence to such "speculative" reports.

Despite the denials that a decision had been made, there were indications that European allies were being informed yesterday that the embargo was about to be lifted.

Gaston Thorn, president of the European Economic Community's commission, received a telegram from Haig informing him of the upcoming action, Agence France-Presse reported from Brussels.

One U.S. diplomat stationed in Europe reacted with dismay. He said that lifting the embargo would undermine efforts to continue to condemn the Soviet invasion of Afghanistan. He also expressed fear that it would strengthen the determination of those in the Kremlin who argue that if they just hang on in Afghanistan and anywhere else, the United States will cave in.

Sen. Charles McC. Mathias Jr. (R-Md.) made public a letter he wrote earlier this week to Haig expressing concern that the administration was considering lifting the embargo.

"I would hope that it is not our intention to reward the absence of bad behavior," Mathias wrote in reference to the administration's reasoning that the embargo could be lifted since the Soviets have not invaded Poland.

In reviewing the grain embargo, administration officials have also been aware of two related deadlines.

Monday is the deadline for preparing the 1981 farm bill.

In addition, a decision on whether to sell grain to the Soviet Union has to be made before Sept. 30, when a five-year agreement under which the United States agreed to sell at least 6 million metric tons of grain annually to the Soviets expires.