Britain today "strongly urged" the warring factions in Zimbabwe-Rhodesia to stop fighting before coming here next month to negotiate a new constitution under which the breakaway British colony could be granted legal independence with a black majority government.

In formal invitations to the constitutional conference at historic Lancaster House here beginning Sept. 10, the British government told Zimbabwe-Rhodesian Prime Minister Abel Muzorewa and Patriotic Front guerrilla leaders Robert Mugabe and Joshua Nkomo that "the prospects for a successful conference will be greatly enhanced if both sides will observe a cease-fire."

There was no immediate response from Muzorewa in Salisbury or Mugabe and Nkomo at their guerrilla bases in neighboring Zambia and Mozambique, but Muzorewa called a special Cabinet meeting for Wednesday to discuss the constitutional conference.

Muzorewa and the Patriotic Front leaders are being pressured to attend the conference by Britain and the "front-line" black African neighbors of Zimbabwe-Rhodesia, who agreed on the independence plan at the British Commonwealth conference in Lusaka, Zambia, earlier this month.

Muzorewa's recently installed multi-racial government would not be expected to survive without British and international recognition and the lifting of economic sanctions against Zimbabwe-Rhodesia. The Patriotic Front rebels, meanwhile, depend on sanctuary and support from the front-line nations in their struggle against Salisbury.

Britain said in its invitation to the conference that "the need for a political settlement and an end to the war in Rhodesia is more urgent today than it has ever been." Both sides were asked "to approach these negotiations in a constructive and forward-looking spirit and to lay the foundations for a free, independent and democratic society in which all Rhodesians, whatever their race or political beliefs, will be able to live in security and at peace with each other and their neighbors."

Accompanying the invitation was an outline of British proposals for a constitution for the independence of Zimbabwe-Rhodesia, based on the constitutions of other former British colonies.

Although purposely vague on details that are to be negotiated here next month, the outline suggests that Zimbabwe-Rhodesia's minority of 250,000 whites still be reserved an undetermined number of seats in Parliament. But under the proposal, they would not have power to block legislation or constitutional changes as they do now.

The British also suggest constitutional provisions that would increase the influence of Zimbabwe-Rhodesia's nearly 7 million blacks in the military, police, courts and civil service.

Control of these institutions has been maintained effectively by the white minority under the "internal settlement" constitution that put Muzorewa in power with the help of former prime minister Ian Smith, the white leader who illegally declared the colony's independence from Britain in 1965.

But the British postponed -- until after a new constitution is agreed on -- consideration of the crucial question of what happens to the armies of the Salisbury government and the Patriotic Front during a transition to legal independence. Neither side wants the other to be in military control of the country, especially during the transition period when new elections are being held.

The conference invitations and the outline of constitution proposals also are silent on how the British government would supervise and guarantee the integrity of the new elections. Prime Minister Margaret Thatcher, who promised in Lusaka that the British would supervise the elections, has ruled out the use of British troops.

British officials said today they were proceeding on a step-by-step plan in which the first step would be to try to win agreement on a new constitution. Then, they said, problems of transition would be tackled.

Muzorewa and the Patriotic Front leaders were asked to send as many as 12 representatives each to the constitutional conference. They can choose whomever they wish, so Ian Smith could be chosen by Muzorewa as one of the white minority representatives in his delegation.

If Smith came, he would be entering Britain for the first time since declaring Rhodesia's unilateral independence in 1965. The British government would have to issue a special order giving him immunity against being taken to court by any British citizen on charges of rebelling against the crown.

If agreement can be reached on a new constitution, control of the military, questions of transition and the mechanics of holding a new election, Britain would be expected to return Zimbabwe-Rhodesia briefly to colonial status and then grant it legal independence as Zimbabwe.

Lancaster House, an ornate Georgian mansion between Buckingham Palace and St. James Palace on the mall in the royal heart of London, has been the scene of negotiations of constitutions for other former British colonies, as well as a variety of international conferences.

Britain's foreign secretary, Lord Carrington, who helped persuade Thatcher to abandon her previous unconditional support of Muzorewa's internal settlement government in the face of overwhelming black African opposition, will chair the constitutional conference.

Muzorewa will face difficult problems of form and substance in trying to negotiate an agreement. The conference invitation sent to Salisbury, for example, was addressed simply to Bishop Muzorewa in deference to Patriotic Front objections of sitting down at the same table with Muzorewa as prime minister of Zimbabwe-Rhodesia.

The accompanying outline for constitutional suggestions was cautiously worded on the subject of reducing white control of public security and the civil service to avoid frightening the whites or neighboring South Africa, whose white minority government might try to jeopardize the Zimbabwe-Rhodesia peace initiative if its security seemed threatened.

A similar constitutional conference in Geneva in 1976 ended in failure and subsequent efforts by Britain and the United States to negotiate an all-parties settlement in Zimbabwe-Rhodesia have foundered. The guerrilla war has taken the lives of an estimated 8,000 rebels, 8,000 black civilians and 1,000 members of the Salisbury security forces.