Britain today significantly modified its plan for a cease-fire in the Zimbabwe-Rhodesia civil war in an attempt to reassure both the present Salisbury government's forces and the Patriotic Front guerrillas that neither would gain the upper hand.

British officials hoped that the concessions made to meet the concerns of Patriotic Front leaders Robert Mugabe and Joshua Nkomo would win their acceptance of the cease-fire plan which is the last remaining element to be negotiated.

An agreement on the new constitution along with arrangements covering the transition to a black majority rule already have been reached.

In a detailed expansion of the British proposals at the Rhodesian peace conference here, Lord Carrington, the conference chairman and Britain's foreign secretary, promised the Patriotic Front that a monitoring force of British Commonwealth troops would be expanded if necessary to ensure that both sides observe the cease-fire.

British sources said an unnamed Commonwealth country in Asia will be asked to add troops to the monitoring force. This is being done in response to Patriotic Front complaints that the presently envisioned force of troops from Britain, Australia, New Zealand, Fiji and Kenya would be predominantly white.

Carrington also said the monitoring force would remain in the country until a new government has been elected under the supervision of the British, "its authority has been accepted and independence has been granted." He said the British would help in planning for the present Salisbury military forces and the Patriotic Front guerrillas to be merged into "a single army" of "an independent Zimbabwe"

Carrington assured the Patriotic Front that the Salisbury forces would be expected to "disengage" and return to their bases to be monitored during the cease-fire if the guerrilla forces would assemble under the "auspices" of the monitoring force to be similarly monitored, as well as housed and fed.

But Carrington warned that "a token assembly would not fulfill the obligation of a cease-fire agreement." Guerrillas remaining in the field would be considered "unlawful," he said.

This was added to the cease-fire plan to reassure the Salisbury government of Prime Minister Abel Muzorewa, which had already announced its acceptance of the British proposals. Sources said Muzorewa's delegates here had expressed concern that the guerrillas would continue infiltration into the countryside to intimidate voters in a new election and carry on the civil war if the Patriotic Front lost the election.

Mugabe and Nkomo, however, have told the British they believe they would win a fair election. What they then fear, they have said, is an attempted coup by present Salisbury government forces or an invasion by South Africa. This is why members of their delegation repeated to reporters, again today that they fear having their guerrilla forces rounded up during the cease-fire into the assembly places proposed by the British.

Carrington reiterated today that the Salisbury forces would be kept away from the guerrilla's assembly areas, where the guerrillas would be under the authority of their own commanders and able to keep their arms. The commanders of the Salisbury and Patriotic Front forces would sit jointly on a cease-fire commission responsible to the British governor until a new black majority Zimbabwe government is elected.

Mugabe and Nkomo reserved judgment on the plan today and British officials interpreted this as an encouraging sign.