Britain's colonial governor here today accused guerrillas loyal to Robert Mugabe of serious violations of the country's cease-fire and indicated that their political party may be banned from next month's elections if the actions continue.

Lord Soames charged Mugabe's Mozambique-based organization with "substantial" border crossings, sending noncombatants to assembly camps intended only for guerrillas and ordering other troops not to assemble in order to intimidate voters.

All these allegations are violations of the Rhodesian peace agreement signed last month in London by Britain, the Salisbury administration of Bishop Abel Muzorewa and his bitter opponents, Mugabe and Joshua Nkomo, coleaders of the Patriotic Front.

In a statement issued by his spokesman, the governor warned, "we shall insist on strict compliance with these agreements by all parties wishing to take part in the election.

"The agreements which will be enforced are the agreements which were signed, and not the agreements which this or that party might have preferred," he added in a reference to complaints by Mugabe that the seven-day assembly period was too short.

The charges represented a sharp escalation in the British colonial administration's criticism of the actions of Mugabe's Zimbabwe African National Union (ZANU) and its military wing, known as ZANLA. It was the first time Soames, or any other British official, has publicly rebuked any Rhodesian nationalist leader.

Although the statement did not mention Mugabe by name, there was no question that he was the target. Reacting to attacks on his administration by several politicans, Soames said, "it is particularly hard to be criticized by one of the party leaders in view of the performance of some of his supporters."

He then went on to list the alleged military violations by ZANLA and complained that ZANU was still holding political prisoners in Mozambique and "broadcasting inflammatory messages from there."

The attack on Mugabe was part of a detailed six-page progress report issued by Nicholas Fenn, the governor's spokesman, evaluating the situation in the former breakaway colony one month after Soames took over his caretaker role to guide the country to election of a black-majority government.

Soames maintained that significant advances had been made since he took office Dec. 12 in:

Implementing the cease-fire;

Improving relations with long-hostile neighbors who supported the guerrillas during their seven-year war;

Easing restrictions on freedoms imposed during 14 years of illegal independence;

Preparing for the return of about 200,000 refugees;

Laying the groundwork for the elections to be held the end of next month;

Reviving the economy, long dormant from the effects of international sanctions which have now been removed.

Emphasizing that his was a caretaker government, Soames said he had no intention " to preempt the decisions of the elected government or to impose upon Rhodesia any particular pattern of change." The "overriding priority," he said, "is to secure and maintain the cease-fire and to make possible free and fair elections."

"By any objective criterion these measures represent a formidable catalogue of progress. Not bad for the first month of a caretaker government," the statement said.

Specifically, he said, "the initial stages of the cease-fire have been encouraging," with 21,370 guerrillas having assembled by last night.

He cited the reopening of borders and the resumption of air services with Zambia and Mozambique. Just today the main route between Salisbury and Lusaka, Zambia, was reopened with resumption of traffic on the Chirundu Bridge over the Zambezi River.

The governor also defended his controversial decision to retain South African military units at Beitbridge to guard the railway and road connection to South Africa which he c alled "a vital communications link."

The use of Rhodesian security forces to support the police in bringing in recalcitrant guerrillas, criticized by the Patriotic Front, was in accordance with the London agreement and was justified, he said, because the police were thinly stretched.