Former Prime Minister Ian Smith threatened, today to try to use the current power whites have to block constitutional changes to prevent implementation of a new constitution agreed to by Britain and the Salisbury government.

Smith, who left the Salisbury delegation to the current talks in London, disassociated himself from Prime Minister Abel Muzorewa's acceptance of a British-sponsored constitution that would strip Zimbabwe-Rhodesia's white minority of its remaining holds on power.

Muzorewa, who is black, has accepted the document, with the reservation that London drop its economic blockade of its former colony, which unilaterally declared its independence in 1965 under Smith's leadership.

The Patriotic Front guerrillas, with whom the biracial Salisbury government is locked in a bitter war, have yet to give their final word on the document but have chafed under what they term a British "ultimatum."

Under the present constitution, whites control 28 seats in Parliament and can block any changes in the constitution that established the biracial government Muzorewa now heads.

The British proposal would remove this blocking power and also end white control over such key areas as the military, civil service and judiciary. Whites would still be guaranteed 20 percent of the seats in Parliament, even though they make up only 3.5 percent of Zimbabwe-Rhodesia's population.

Smith's use of the white blocking power to veto the proposed constitution would make the lifting of sanctions and international recognition, which Smith has long sought, virtually impossible. It would put the whites in the position of using the blocking power to prevent implementation of a constitution in which a key feature is the removal of just this power.

There is some question just how much power Smith still has over the white political machinery. Whites inside and outside the London delegation have been increasingly critical of his lone stand against the British-proposed constitution.

One key white delegate recently described Smith as a "frustrated, embittered" man who "has done his job and should now bow out."

Smith, who returned to Salisbury Saturday night, was the only member of Muzorewa's 12-man team to vote against the British proposals and he made it clear that he did not feel himself bound by the majority decision.

"I made it clear I did not accept them. I don't accept something that is dependent on something else happening in the future. I want to see the whole deal clear," Smith said.

Smith said he parted company with the British proposals in two key areas -- the control of the security forces and the judiciary.

Smith said it would be "madness to tamper in the fields of law and order" and accused Britain of doing precisely that with its latest proposals.

"I would say to you that clearly these things are unacceptable, and unless they are changed these British proposals will be rejected by our Parliament," Smith warned, adding that they were "worse for us" than any previous British offers.

Accusing British negotiators of being "demanding and even unreasonable," Smith had some harsh words for Lord Carrington, the foreign secretary and chairman of the constitutional conference, whom he accused of forcing a split in the Salisbury delegation.

Smith is to meet Monday with the white parliamentarians, reduced to 27 by the recent murder by guerrillas of one of their number. The white bloc two weeks ago sent a telegram to Smith congratulating him on his stand against the removal of current safeguards for whites.

In terms of the present constitution at least six whites will have to vote with all 72 black members of Parliament to pass constitutional amendments removing present white control over the key areas.

Smith, by breaking with his Cabinet colleagues, has clearly lost the support of the three other whites in the Salisbury delegation to London -- David Smith, the finance minister, Chris Andersen, the justice minister, and Rowan Cronje, the deputy minister of lands.

Monday's meeting of the Rhodesian Front caucus will reveal whether another three white parliamentarians are prepared to break with Smith and offer Muzorewa the votes he will need to introduce a new constitution.

Although Smith conceded that there is disagreement within the Rhodesian Front over the British proposals, he expressed confidence that enough would side with him to prevent Muzorewa's pushing through a new constitution. In the 15 years since he took over the party, he has proved adept at holding the caucus together.

Smith alleged that the new constitution would give a prime minister dictatorial powers over the appointment and dismissal of security chiefs and judges and claimed that "if he should wish to go outside our country, for example to Russia or one of its satellites to recruit replacements for these vital positions, then he may do so."