Britain presented a detailed proposal for a constitution for Zimbabwe-Rhodesia today, posing a dilemma for the Patriotic Front guerrilla alliance to either agree or risk responsibility for the breakdown of the 24-day-old Rhodesia settlement conference.

British Foreign Secretary Lord Carrington told the warring Zimbabwe-Rhodesian government and the Front that Britain felt the draft constitution was fair and the government intends to recommend the document to Parliament as the basis for granting legal independence.

British officials denied that the proposal, which has been "exhaustively" discussed for the last three weeks, was an ultimatum to Front leaders Robert Mugabe and Joshua Nkomo or to Zimbabwe-Rhodesian Prime Minister Abel Muzorewa.

However, the implication that the British may forge ahead with the Muzorewa administration alone was certainly not lost on Mugabe and Nkomo, who are still objecting to a number of points in the British draft.

Thus, the British move -- the ninth effort during the last 14 years to solve the intractable Rhodesian problem -- brought the matter to a critical point. The problem has been complicated by a worsening guerrilla war.

British spokesman Nicholas Fenn maintained that the document presented today by the government is a compromise between the opposing stands of the guerrillas and the Salisbury administration, which has the support of the 230,000 white minority.

The British proposal removes the parliamentary blocking power whites currently enjoy in many areas of government and also denies them control of key commissions that have considerable authority over government activities.

On the other hand, it provides the 3.5 percent white minority with 20 percent of the seats in the lower house of Parliament and protects that right from constitutional amendment unless it receives unanimous approval of the assembly.

Former Prime Minister Ian Smith, who declared illegal independence from Britain in 1965, led what ended up being a lone battle for the blocking power but his tactics undoubtedly played a role in the British granting significant safeguards to the whites.

A key area of white protection involves a declaration of rights that prohibits seizure of private property without compensation. The Front opposes this proposal, maintaining that whites, who own most of the best land, originally seized it from blacks.

The constitution prevents amendment of the rights declaration for 10 years unless there is unanimous approval by the assembly.

Lord Carrington said in a television interview tonight that he thought a breakdown of the conference was unlikely but added "the time has come to make up our minds."

Most observers feel the Front will eventually give way on the constitutional issues in order to move on to the second part of the conference dealing with a transitional government to transfer power, which is the key concern of the guerrillas.

An overall settlement holds the prospect for ending economic sanctions against Salisbury, granting of international recognition and removal of a major problem for Western relations with Africa. But an agreement that excludes the Communist backed Patriotic Front would almost guarantee an escalation of the war, already Africa's bloodiest independence struggle, and the potential for serious East-West involvement.

Carrington adjourned the conference until Monday, inviting both delegations at that time to consider "and, I trust, to accept" the proposed constitution.

Both rival delegations declined any public comment until they had studied the 34-page document.

Other major areas of difference between the British and the Front include the issues of citizenship, pensions and the nature of the executive.

Most observers feel that the guerrillas, having already conceded on the major issue of white representation, also will give way on these lesser issues.

With their bases in Mozambique and Zambia, the guerrillas are subject to pressure from neighboring African nations that are suffering from the escalation of a war that has already killed more than 20,000 people in Zimbabwe-Rhodesia and badly disrupted southern African economies.

The guerrillas have long said that the constitution is less important than the transitional issues, especially control of the military, which is likely to determine who gains power in elections to implement the constitution.

Under the terms of the conference, agreement on the constitution is linked to a similar settlement of the transitional issues but approval of the constitution must come first.