Philip C. Habib, President Reagan's special envoy to Central America, was in error and imprecise in a letter that he wrote about U.S. policy there and that has outraged conservatives, a senior administration official said yesterday.

The official, who briefed reporters on condition that he not be identified, was apparently trying to allay conservatives' fears that Habib is selling out U.S. interests and U.S.-backed antigovernment rebels in Nicaragua in working for the pending Contadora regional peace treaty.

An aide to Rep. Jack Kemp (R-N.Y.), who called Thursday for replacing Habib, expressed satisfaction at the official's remarks. But House liberals said the week's events have probably scuttled any chance that the leftist Sandinista government of Nicaragua might sign the Contadora pact by the negotiating group's self-imposed June 6 deadline.

The developments came as the presidents of the five nations that would sign the agreement -- Nicaragua, El Salvador, Guatemala, Honduras and Costa Rica -- prepared to meet this weekend in Guatemala to work on the thorny and crucial issues remaining.

These include arms limits, enforcement provisions and a timetable for implementation. Proponents of a Contadora treaty argue that it offers the best way to contain the Sandinistas in Nicaragua and democratize the country without committing the United States to a long-term and expensive role on the ground in the region.

Whether prospects of a treaty have improved or decreased is unclear, diplomatic sources said.

But alarm expressed by U.S. conservatives in recent days that Habib was negotiating a sellout of the rebels, called contras or counterrevolutionaries, evidently had the ironic effect at first of increasing Nicaragua's interest in proposed terms for a treaty and in Habib's role.

Habib's letter to a member of Congress April 11 said U.S. aid to the rebels would cease "on signature" of a verifiable pact honored by Nicaragua.

Asked if that letter was "in error" in promising action on signature, the official said, "Yes." He also said it was "imprecise" and said that, if he could rewrite the letter, "I'd just change the word 'signing' to 'implementation.' "

He said there was "a certain impetus, I would say mostly in South America, to finish the treaty process and withdraw from it by signing a document, regardless of the content thereof. We're sure opposed to that."

The official repeated in response to questions that Reagan retains full confidence in Habib. He insisted that the U.S. position on the talks has remained constant "over the past two years" in favoring an enforceable and comprehensive treaty that would require Nicaragua to democratize, lower arms levels, cut ties to Cuba and the Soviet Union and end support for outside guerrilla movements.

The United States, although it is not a party to the treaty, will end support for the contras "simultaneously" with verified Nicaraguan compliance with an acceptable pact, he said.

Conservatives and liberals said they saw a definite shift in the administration stance.

The Kemp letter calling for Habib's recall "woke them up that there is an effort going on that threatens the Reagan doctrine," a Kemp aide said. "The whistle got blown . . . there was a little free-lancing going on."

Conservatives' cries of alarm "halted the inevitability of a treaty that would not meet the president's goals."

He said Kemp still wants Habib recalled on grounds that he is identified with ending aid to the contras as soon as a treaty is signed.

Spokesmen for two House liberals said the furor had shown that the Reagan administration will not support any agreement that might allow the Sandinistas to remain in power in Nicaragua.

Steve Champlin, an aide to Rep. David E. Bonior (D-Mich.), head of the House Democratic task force on Central America, said the administration official "made clear that in fact the administration is reserving the right to unilaterally decide whether the treaty is comprehensive and verifiable and whether Nicaragua is in compliance. . . . If Nicaragua signs and disarms as the treaty requires, its existence would then be at the whim of the Reagan administration."

The Habib letter, he said, had indicated "only a very small movement in policy and was cautious in the extreme." Conservatives who found it threatening "showed that they are not even close to a treaty," he said.

Mark Helmke, an aide to Senate Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.), said Habib retains Lugar's strong support. He said the June 6 deadline "might slip" if there is movement at this weekend's summit talks.

"The danger of all this flap is that the four countries other than Nicaragua might read this as a sign they are being undercut politically" by the Reagan administration, Helmke said.

"They may get cold feet from coming up with a treaty that would be strong and enforceable . . . . It's best to support them and then, if Nicaragua doesn't sign, it is the one on the record that its diplomacy is a sham, not the United States."