Britain and the Patriotic Front guerrillas reached agreement tonight on a cease-fire plan to end the civil war in Zimbabwe-Rhodesia.

While significant details of the plan's implementation must now be worked out in negotiations involving the commanding generals of the opposing forces, tonight's agreement is regarded here as the key breakthrough in the three-month-old peace talks.

"I don't think anybody will turn back now," said British Foreign Secretary Lord Carrington, the conference chairman. "With good will," he said, "and I'm sure after today there is good will, I think we can tie up the details in a few days."

A formal cease-fire document is to be presented by the British Thursday to Patriotic Front guerrillas leaders Robert Mugabe and Joshua Nkomo and to the delegation of the present Salisbury government of Prime Minister Abel Muzorewa.

Once the two warring sides agree on a cease-fire date and precisely how their forces will disengage, this document is to become the final peace agreement. It will be signed by the two sides and by the British at a formal ceremony here, ending the long war and making possible new elections under a British governor to produce a legally independent Zimbabwe with a black majority government.

British and diplomatic sources here still expect tough bargaining by the Patriotic Front leaders on the cease-fire date and the positioning of the rival forces after the cease-fire. The Front is trying to gain as much time as possible for its supporters to continue infiltrating back into Zimbabwe Rhodesia from exile in neighboring African nations.

But everyone today, including Front spokesmen, predicted the cease-fire agreement would be signed sometime next week at the latest. "I don't think there is now anyone who doesn't believe we are now going to have a final settlement," said one well-placed British source. "A political process has begun that cannot be reversed."

The British government already is promulgating the independence con stitution agreed on earlier here and is making arrangements for the British governor to to Salisbury "in the next few days," possibly before the final peace agreement is signed.

This process was begun this week to push the Patriotic Front into accepting the British cease-fire plan after nearly a week of deadlock and ignored ultimatums.

The British also made significant concessions to gain tonight's agreement. A compromise was negotiated in intensive behind-the-scenes negotiations by Shridath Ramphal, the London based secretary genered of the British Commonwealth, which includes African countries who back the Patriotic Front.

Through Ramphal, the British and the Patriotic Front agreed to carry over into the negotiations on the details of implementing the cease-fire the Front's objection to having its forces rounded up into 15 assembly places while Muzorewa's forces remain in far more numerous bases.

The British also agreed to enlarge the force of British and Commonwealth troops monitoring the cease-fire to about 1,200 men to reassure Mugabe and Nkomo that their forces will be safe.

Commonwealth sources indicated tonight that the monitoring force may be enlarged still further and include troops from a Commonwealth country in Asia along with those already allocated by Britain, Australia, New Zealand, Fiji and Kenya.

Finally, Carrington agreed to make a public promise that South African troop now in Zimbabwe-Rhodesia will be withdrawn when the British governor goes to Salisbury. This was the last point to be settled when tonight's deal was sealed at a meeting Carrington called with Mugabe and Nkomo.

Mugabe had been reluctant all along to agree to the compromise being negotiated through Ramphal, according to inmformed sources. They said he now objected that Carrington's proposed statement would not name South Africa, whose prime minister, Pieter Botha, publicly acknowledged last week that its troops had been "actively protecting" trade routes inside Zimbabwe-Rhodesia.

Carrington had intended to state only that "I can assure you again that there will be no external involvement in Rhodesia under the British governor. The position has been made clear to all the governments concerned." g

With Mugabe still not satisfied and Carrington committed to reporting to Parliament within minutes on the progress of the negotiations, Carrington agreed to add "including South Africa" at the end of the statement.

Ramphal, whom Carrington commended for his role in reaching tonight's agreement, also met recently with Prime Minister Margaret Thatcher.She told him, according to informed sources, that she was determined to achieve a final peace settlement and to make it work through the interim British governor in Salisbury.

The silent partner in today's agreement was the Salisbury delegation currently led by Deputy Prime Minister Silas Mundawarara, who had accepted the British cease-fire plan 10 days ago. Mundawarara had become impatient with the Patriotic Front's delaying tactics and yesterday he threatened to take his delegation home.

Tonight, Mundawarara said it was his pleasure to "compliment our brothers" in the Patrotic Front for "this real progress."

Carrington, who has played the heavy during the tense negotiations of the past few days, said the word "brothers" was encouraging. "Nobody had called me their brother," he told the two delegations at the brief formal conference session at which agreement was announced tonight, "but perhaps that is the fate of the chair."