The League of Women Voters yesterday invited independent candidate John B. Anderson to take part in its presidential debates, and President Carter promply told the league he could not participate.

Carter, who had accepted the league's debate invitation without conditions in May, said through campaign manager Robert S. Strauss that he now "respectfully declines" to debate Republican candidate Ronald Reagan one-on-one before he faces the challengers togethers.

Reagan and Anderson both said they would take part in the debate, Carter or no Carter. So, it seems likely that the first nationally televised debate of the 1980 campaign will feature the two challengers but not the incumbent.

In a tarmac news conference at Chicago's Midway Airport, Reagan was asked what he would do if Carter turned down the league invitation. "Well, I'm going to be there," the candidate said. "The ladies have decided that Anderson is a viable candidate, and I've always said if he was, he certainly should be included."

Anderson, campaigning in New Jersey, accepted the league's invitation "without qualification or condition. The League of Women Voters has acted responsibily in inviting me to participate," he said.

Representatives of the Anderson and Reagan campaigns will meet today to discuss the debate's timing and format. If both sides accept the league's proposal, the debate will take place Sept. 21 in Baltimore.

CBS News announced last night it would carry the debate live, and officials at ABC indicated they would likely do the same, despite the president's absence.

The Carter campaign, worried that any boost to Anderson's candidacy will drain votes from the incumbent, has been pressuring the league to open its series of three presidential debates with a confrontation between the Republican and Democratic candidates.

But the league reviewed recent polls yesterday and decided that Anderson has "significant" voter support. According, Anderson was invited to take part in all three planned league debates, and his running mate, Patrick J. Lucey, was inviting to a fourth debate among vice presidential candidates.

Three hours after the league's announcement, Strauss released a statement saying that Carter would not take part. "We are convinced that acceptance of this invitation would preclude any chance of . . . a one-on-one debate [with Reagan], and therefore we must respectfully decline."

But Reagan's chief adviser on the debates, James Baker, said the Reagan camp "probably" would be willing to meet Carter head-to-head sometime -- but not in the first debate of the campaign.

Later, in Milwaukee Reagan, appeared to undercut Baker's position Reagan was asked whether he would agree to debate Carter one-on-one if Carter would agree to engage in a three way debate.

"No, because I don't think it would be any more fair to do it then if there are three viable candidates than it would be to do it now," Reagan replied.

Reagan's press secretary, Lyn Nofziger, later refused a request to clarify Reagan's remark.

The big winner in yesterday's development was Anderson. The league's decision was the third piece of good news Anderson's campaign has received in the last week. First, the Federal Election Commission ruled that he should be eligible for retroactive federal matching funds if he gets at least 5 percent of the vote in November. Then the policy committee of New York state's Liberal Party endorsed him, virtually assuring him a line on the New York ballot. Together, the three events have given his faltering campaign a boost for the first time in months.

Carter's campaign manager, Strauss, has complained that Anderson's presence will "dilute" the president's efforts to show that he is better qualified for the White House than Reagan. The Carter campaign's central strategy has been to play on doubts about Reagan's ability, and to ignore Anderson.

During a campaign trip to New Jersey yesterday, Carter said that Anderson is "primarily a creation of the press."

"He hasn't had a convention, he doesn't have a party," the president explained. "He and his wife handpicked his vice presidential nominee. But . . . he's given equal treatment on the evening news and in the newspapers . . ."

Carter added that "a three-person debate format is more like a forum than it is a real debate."

The league invited the major party candidates to participate when it first announced its plans for a series of debates this fall. It said than that it would consider other candidates for inclusion if they were constitutionally eligible for the presidency, were on the ballot in enough states to win an electoral college majority and "have demonstrated a significant measure of nationwide voter support and interest."

To determine whether a candidate has "significant" support, the league established the requirement of 15 percent support in national opinion surveys.

A five-member league committee looked at eight different polls taken over the last two months by five different organizations, but paid closest attention to the most recent four.

They were a Los Angeles Times poll taken Sept. 2-7, in which Anderson was backed by 18 percent of those surveyed, a Harris poll taken Sept. 3-7 him at 13 percent, and a Time magazine poll taken Aug. 26-28 that gave him 15 percent.