The British government said today it was all but helpless to avert mounting bloodshed in Rhodesia, at least for now.

Prime Minister James Callaghan told the House of Commons he had accepted the advice of his special envoy against any attempt to bring together Prime Minister Ian Smith and his black allies with the guerrilla leaders seeking to overthrow them.

The two sides "are very far apart," Callaghan said. "There is at present no possibility... to allow hope that agreement might be reached."

Callaghan implied that President Carter shared his gloomy view, noting that he had discussed his envoy's report with the president at the summit meeting in Guadeloupe.

The report by the envoy, member of Parliament Cledwyn Hughes, was made public today. Hughes observed that Stephen Low, the U.S. ambassador to Zambia, "fully supports the conclusions." Low had accompanied Hughes on his futile mission in November and December to Rhodesia and seven other African nations.

The formal abandonment of a British-sponsored peace conference is no surprise. But London's admission of virtual impotence over the course of affairs in what is still legally a British colony is unusual.

Diplomats here take satisfaction only in the fact that Anglo-American representatives can still talk to both sides -- to Smith, Bishop Abel Muzorewa and their team as well as to Joshua Nkomo and Robert Mugabe, leaders of the loosely allied guerrilla forces.

But the new U.S. Congress, it is feared, might change even this. Diplomats think the Congress may force Carter to end trade sanctions against the Smith government and thereby alienate the black guerrillas and the black African states supporting them.

As the law now stands, experts here point out, sanctions must be lifted if Smith has made a genuine effort to negotiate with the guerrillas and if he holds free and fair elections for the National Assembly.

The diplomats are worried that Congress, absent such supporters of black African aspirations as former Democratic senator Dick Clark of Iowa, will insist that the conditions have been fulfilled -- provided Smith keeps to his schedule for a vote on April 20.

Hughes said he believed Smith and the others could be brought to a conference table but their talks would go nowhere. "Each side in the war is convinced that it can reach its own goal -- or at least not lose -- by continuing to follow its own present policies," he said.

Each side, Hughes said, would simply seek "to demonstrate that the other is unreasonable and intransigent" in order to win support from abroad.

A conference that failed, Hughes continued, would strengthen the determination of each side to go its own way, damage British relations with Africa and bring "heavy pressure" from black Africa and the United Nations to impose trade sanctions against South Africa, supply British arms to guerrillas and even bring calls for a British military force to intervene.

Moreover, Hughes warned, a deadlocked conference would drive the Nkomo-Mugabe forces and neighboring African states closer to the Soviet Union and Cuba.

The immediate prospect, Hughes made clear, is grim. "More and more men, women and children are being killed or maimed... Everything suggests that the situation will get worse, not better."

If this compels Smith and his allies to postpone the scheduled assembly elections for a second time -- they had originally been promised for the end of 1978 -- they might be willing to make fresh concessions. But this "would no doubt" lead the guerrilla forces to step up their demands.