Patriotic Front leaders Robert Mugabe and Joshua Nkomo, under growing pressure to accept British proposals on a constitution for an independent Zimbabwe, charged today that Britain and the Zimbabwe-Rhodesian government had made a secret deal.

The charge, made after the first plenary session of all three delegations in two weeks, threw into confusion the British-sponsored conference seeking to solve the 14-year-old Rhodesia question.

British sources denied there had been secret talks. They indicated that the problem, which they described as "not very serious," could be cleared up as early as tomorrow, when Britain hopes to present revised proposals for a new constitution.

Zimbabwe-Rhodesian Prime Minister Abel Muzorewa said he was puzzled by the Patriotic Front tactics. Other delegates accused the guerrilla leaders of being insulting.

Mugabe and Nkomo gave vent to their criticism in their first press conference since the talks started 23 days ago. They were accompanied by Mugabe's military chief Josiah Tongogara, making a rare public appearance and thus drawing more attention to the controversy.

After today's 70 minutes of talks -- followed by press briefings of similar length -- it was clear that no progress was made.

The Front's charge of a secret deal came after they tried unsuccessfully to get the Muzorewa delegation to join in discussion on points the guerrillas want included in the constitution but the British have rejected.

Muzorewa declined, saying he had already accepted the general principles of the British plan during bilateral talks, and was waiting only to see the "refined" proposals.

Today's flap came amid reports that the guerrillas were under growing pressure from the neighboring African front-line states to reach agreement on the constitution to move on to the transitional issues.