The British government today said it will propose a new plan for restoring Zimbabwe-Rhodesia to legality and making the multiracial government of Prime Minister Abel Muzorewa acceptable to the rest of the world.

Prime Minister Margaret Thatcher's foreign secretary, Lord Carrington, announced that, as a result of preliminary consultations with African leaders, Britain intended "to make firm proposals of our own to bring Rhodesia to legal independence on a basis which we believe should be acceptable to the international community."

Those proposals would include changes in Muzorewa's government and constitution to reduce the power reserved for Zimbabwe-Rhodesia 3 percent white minority, according to British government officials. They said Thatcher and Carrington will urge Muzorewa in meetings here later this week to seek changes in constitutional provisions preserving white control over the military, police, civil service, judiciary and the process for amending the constitution.

Lord Harlech, Britain's special emissary to Zimbabwe-Rodesia's black African neighbors and Nigeria, found that some of their leaders might accept Muzorewa's government if sufficient constitutional changes were made, according to one well-informed British official. The official would not name these leaders.

"they don't rule Muzorewa out if he demonstrates that he is in charge and makes those changes," the official said. "Some of them acknowledged that there had to be some special protection of whites -- as there was when they were granted independence from Britain -- but they felt the Rhodesian constitution just went too far."

Officials here say Muzorewa apparently does not believe that he can get the necessary support from the whites to make the constitutional changes. But the officials add that the British Foreign Office has received an informal report that some influential whites were ready to agree to changes, so long as their security was not compromised, and were waiting for Muzorewa to ask them.

British officials have little hope that the plan they devise will be acceptable to the Patriotic Front rebels led by Joshua Nkomo and Robert Mugabe, who continuing their guerilla war against Muzorewa's government from bases in neighboring Zambia and Mozambique. One source said Britain's goal would be to win enough African and other international support for the British plan to isolate the rebels and force their defeat or repatriation.

It is also clear that Thatcher's Conservative government remains committed to Muzorewa as the democratically elected black leader of Zimbabwe-Rhodesia and determined to find a way to give his government legal recognition.

Thatcher said in a press conference in Australia last month and privately in a meeting with U.S. Sen. Jesse Helms (R-N.C.) last week that Britain's 13-year-old economic sanctions against Zimbabwe-Rhodesia will be ended when they come up for renewal in Britain's Conservative-controlled Parliament in November.

She is under heavy pressure form the right wing of her party in both the House of Commons and House of Lords to end sanctions and move more quickly to give Muzorewa's government legal recognition.

But her commitment to restore Zimbabwe-Rhodesia to legality has been publicly criticized by much of black Africa and the rest of the British Commonwealth. The Organization of african Unity summit meeting next week in Monrovia, Liveria, is bound to produce more criticism.

Thatcher could face additional problems when the commomwealth nations meet next month in Lusaka, Zambia, the base for Nkomo's guerrillas. Lusaka has suffered frequent military attacks from Zimbabwe-Rhodesia at the conference, which both Thatcher and Queen Elizabeth II will attend, could lead to the breakup of the Commonwealth. Britain's economic ties with Nigeria, its biggest trading partner, also are threatened.

Thatcher told Helms, the chief supporter of the Muzorewa government in the U.S. Senate, that she was afraid of the Commonwealth meeting, according to informed sources. She told Helms that she wanted to convince enough black African nations to support the British plan for recognizing Zimbabwe-Rhodesia to win over a majority of the Commonwealth members.

Although the British plan will not be completed and announced until after the Commonwealth meeting, Thatcher will be consulting with the other heads of government in Lusaka on its likely contents. Her government's hope to have the most acceptable plan possible ready before the economic sanctions come up for renewal in Parliament here in November.

In his announcement today of Britain's intentions, Carrington made no mention of the United States. The previous Labor government here and the Carter administration had jointly drawn up a settlement plan for Zimbabwe-Rhodesia that failed to gain acceptance. British government officials said the United States would be consulted but that the plan likely would be solely british.

U. S. officials, attempting to forestall attempts by conservatives in Congress to force recognition of the Muzorewa government, have emphasized in recent months that the problem is basically a british one now and the next move was up to Britain.

In making his announcement in a speech in the House of Lords here today, Carrington noted that Britain did not share the "disposition in some sections of the international community to seek to ignore the changes which have taken place in Rhodesia to argue that Bishop Muzorewa should be treated in the same way as the previous government of Ian Smith."

But Carrington said that Lord Harlech, in his consultations with government leaders in Nigeria, Zambia, Tanzania, Botswana, Malawi, Angola and Mozambique, "found in Africa encouraging recognition that major changes had taken place" in Zimbabwe-Rhodesia with the election by black and white voters of a multi-racial government.

He said they also made clear, to Harlech their criticisms of the new Rhodesian constitution and their "recognition and indeed even a demand that a solution must spring from Britain, as the legally responsible authority" for its former colony.

Carrington said Harlech also met with representatives of the Partiotic Front and communicated "the views and attitudes" of the African presidents and the Patriotic Front to Muzorewa. He also conveyed Britain's "conviction that the present situation gives us the opportunity to work with him for a lasting and generally acceptably settlement," Carrington said.

Carrington said he believed the British government, through its new contacts with the Muzorew government, is now "in a better position to influence events in Rhodesia than at any time" since Ian Smith declared white run Rhodesia's unilateral independence in 1965 and was outlawed by Britain and the world.

Many observers feel that Muzorewa's chances to maintain power are decreasing markedly as the gurrilla war continues and he fails to gain recognition. Last month, Muzorewa lost his absolute majority in the Rhodesian Parliament with the defection of an eight-member bloc led by James Chikerema.

White emigration, which had slowed earlier this year, has increased again to about 1,400 a month. Since whites form the back bone of the military and police forces, a continuing exodus could doom Muzorewa no matter what Thatcher does.

Another potentially crucial test will come next month when the spring planting season begins. There is growing concern that many white farmers will cut back or stop planting because of fears for their safety, which would be another bad blow to the already weakened economy.