Prime Minister Abel Muzorewa, already under pressure from forces outside Zimbabwe-Rhodesia, suffered a blow on the domestic political front today when a court deprived him of his parliamentary majority.

The court of appeals in Salisbury rejected a bid by Bishop Muzorewa to oust from Parliament seven members who broke with his United African National Council last month. The decision effectively cuts Muzorewa's seats in Parliament from an absolute majority of 51 to 44 in the 100-seat legislature.

In the complex politics of embattled, Zimbabwe-Rhodesia, Muzorewa's ruling party could be outvoted if the combined opposition forces, including the whites, united against him. In any case, white influence seems certain to be increased by today's ruling as the 28 white members of Parliament are now crucial for the bishop to maintain his authority.

This weakening of Muzorewa's position could in turn have a significant effect on Britain's delicate diplomacy, which is seeking to change Zimbabwe-Rhodesia's constitution to facilitate international recognition of his government.

Britain reportedly is pressing Muzorewa to drop Smith from the Cabinet and ease white control in such areas as the military, police, civil service and the judiciary.

Muzorewa has resisted the pressure, citing the need for the 230,000 whites to remain in the country since they form the backbone of the military, which is fighting the 6-year-old war against guerrillas seeking full power for the 6.5 million blacks.

The victor in the court battle was James Chikerema, Muzorewa's former lieutenant who has vowed to replace him. Chikerema led the seven members of Parliament out of Muzorewa's party last month.

Muzorewa maintained that the seven seats should be declared vacant since the members were elected on a party slate rather than as individuals. The all-white appeals court refused, however, saying it had no power to determine representation in Parliament.

The decision means that Muzorewa's party, which took control May 28, has 44 seats, compared with 28 for Smith's all-white Rhodesia Front. If the front combined with Chikerema's seven and the nine members of a tribal-based, white-influenced minor party the two sides would each have 44 votes.