Iran's military-led government adjouned parliament today in an apparent effort to avoid a barrage of criticism and facilitate attempts to form a new civilian Cabinet under Shah Mohammad Reza Pahlavi.

The man tapped by the Shah as the new prime minister, Gholam Hussein Sadighi, 73, is having difficulty finding candidates for a civilian government and is given only a slim chance of being able to form one, diplomatic sources said. They said the process, if successful, would probably take two or three weeks and this figured in the decision to adjourn the parliament.

The essentially rubber-stamp legislature had been expected to start debating three censure motions against the 45-day-old militay-led regime on Dec. 24. But the government announced that winter vacations will begin early this year and that the body will adjourn Dec. 23 for three weeks.

Though the debate could hardly be expected to result in actual passage of any censure motion, it would have provided a plateform for the handful of opposition deputies to further condemn the government and show up the rest of the parliament as unrepresentative of popular opinion.

In a letter made public today, the leader of the opposition National Front, Karim Sanjabi, denounced what he said was Sadighi's acceptance of the shah's request to form a government. Sadighi and Sanjabi formely were allies in the National Front govrnment under the late Mohammed Mossadegh, who was ousted in a CIA-backed coup in 1953 that restored the shah to the throne.

"Your decision to join the present regime would only result in failure, and therefore you cannot depend on any consent or cooperation of the Iranian people and the National Front," the letter said.

In an oral reply passed on by a friend, Sadaghi denied that he has agrreed to form a new government. His daughter, acting as his spokesman, bitterly denounced Sanjabi and said his letter was not approved by the National Front's executive council.

She said other senior National Front members, notably Dariush Foruhar, had visited Sadaghi to express regrets over the letter. Foruhar is considered the number three man in the front.

In a separat letter addressed to "countrymen," Sanjabi disavowed any connection between Sadaghi and the National Front, which he said "would not agree to any coalition under the present illegal monarchical regime."

Sadaghi, who quit the front in a dispute with Sanjabi in 1963, in insisting that the shah agree to "certain conditions" before accepting the premiership, his daughter said.