The British government has complained to the United States that diplomatic lobbying efforts by two staff members of Sen. Jesse Helms (R-N.C.) have impeded the current attempt to negotiate a settlement of the civil war in Zimbabwe-Rhodesia, State Department and congressional sources said yesterday.

Helms, who called the charges "a perfect absurdity," said he was called to a conference on the matter by the chairman and ranking Republican on the Senate Foreign Relations Committee at the behest of Secretary of State Cyrus R. Vance.

Helms said he did sent two aides, John Carbaugh and James P. Lucier, to the London conference on the Zimbabwe-Rhodesia settlement last week because "I don't trust the State Department on this issue." But he denied that they engaged in lobbying to "stiffen the spine" of former Rhodesian prime minister Ian Smith, who is a holdout against proposed terms of a settlement. "They didn't need to" stiffen Smith's spine, and the senator.

The trip of the two aides was paid for by a private right wing foundation, the Institute of American Relations. Helms said this was done with his permission and that his staff members "went on vacation" to make the journey with no government involvement.

According to State Department officials, Vance received a "very serious complaint early yesterday from Lord Carrington, the British foreign secretary, that the two Helms aides had interfered at a crucial point in the delicate negotiations.

The British are reported to believe that Smith's unyielding position against the proposed settlement arises in part from suggestions by the Helms aides that Congress will lift sanctions against Zimbabwe-Rhodesia and thus ease that nation's plight, if Smith will hold fast for the time being.

A conference committee of the Senate and House Armed Services committees, meeting on the defense authorization bill, is scheduled to convene today to begin consideration of an amendment that would lift the sanctions against Zimbabwe-Rhodesia immediately. Such a decision at this time by the conference might torpedo the chances for a Zimbabwe-Rhodesia settlement along the currently suggested lines, according to diplomatic observers.

Helms aide Carbaugh while in London to monitor the Zimbabwe-Rhodesia negotiations made no secret of his prediction that the Senate-House conference committee would vote to lift sanctions by early October, according to Washington Post correspondent Jay Ross.

After receiving the British complaint, Vance telephoned Chairman Frank Church (D-Idaho) and Sen. Jacob K. Javits (R-N.Y.) of the Foreign Relations Committee to discuss the matter. Church and Javits then met with Helms in a Senate cloakroom.

"I told Church and Javits if they had any idea they could intimidate me, to forget it," Helms said.

Neither Church nor Javits could be reached for comment.

Helms, a strong backer of the white cause in Zimbabwe-Rhodesia and an advocate of the removal of sanctions, said his aides "may have given a report" on the status of the sanctions-lifting amendment during their visit to London. He insisted, though, that "at no time did they lobby anybody."

State Department spokesman Hodding Carter said he would have no comment on U.S. legislators who display interest in foreign policy matters as long as they operate within U.S. law. While maintaining he had nobody particular in mind, Carter added it would be "unfortunate" if anyone acted in a way that would give aid and comfort to those who would wreck a negotiated settlement of the Zimbabwe-Rhodesia civil war.

Interviewed by telephone last night, Lucier said he and Carbaugh make up the board of directors of the Institute of American Relations along with its executive director, Victor Fediay. Lucier said the two Senate aides received no pay from the institute but have traveled abroad at its expense to several countries, including Zimbabwe-Rhodesia and South Africa. He said they are reimbursed for Washington expenses.

Carbaugh said he and Lucier were among the founders of the 4-year-old institute. Both aides said no foreign money is involved in financing the organization.

Lucier added he could understand what the British complaint is. "Sure we talked to Smith," Lucier said. "We both talked to him. He told us his opinion and we told him" the conference committee would finish work on the sanctions amendment soon. "We told him odds were that the conference would accept" the amendment, Lucier said.

Asked if he thought the conversation encouraged Smith, Lucier replied, "Ian Smith is a man who has held out against the world for 14 years. It's ridiculous to think a man like me could go over there and change his mind about everything."