Lewis A. Tambs, an outspoken conservative who opposed the Panama Canal treaties, said yesterday in London that the Reagan administration has reversed itself and decided not to name him ambassador to Panama.

Tambs, backed for the post by Sen. Jesse Helms (R-N.C.), said the White House told him late Tuesday that he would not get the job. He claimed President Reagan personally had offered him the post earlier, although the nomination never was official.

Tambs, a professor in Latin American history at Arizona State University, said he had reminded the president that he, like Reagan, had opposed the Panama Canal treaties. Reagan, according to Tambs, said he didn't care.

"I understood that the Panamanians wouldn't object to whomever the president chose," Tambs, who was visiting London with a group of U.S. conservatives, told newsmen.

The on-and-off Tambs nomination is only the latest in a series of efforts by Helms and other conservatives to place philosphical soulmates in foreign policy posts. These efforts have set up divisive confrontations with moderates in the Senate and the State Department.

Congressional sources said that Secretary of State Alexander M. Haig Jr. moved to block Tambs' appointment after being told by senior Republicans that the Senate Foreign Relations Committee would refuse to confirm him because of his opposition to the Panama Canal treaties.

"Putting him down there would be like igniting a powder keg," said one well-placed Senate source.

Tambs testified against the treaties, approved in 1978, and edited a paper highly critical of them for the Reagan transition team. Helms earlier pushed him as a conservative candidate for assistant secretary of state for inter-American affairs.

Tambs, who has sought other appointments, has expressed outspoken views on a host of foreign relations issues. He has criticized detente and the SALT II treaty, endorsed closer relations with South Africa and described the Caribbean as "a Marxist-Lenin lake."

In a 1981 interview with the Tempe (Ariz.) Daily News, he said U.S. foreign policy must recognize Latin America "as the soft underbelly of the United States."

"If propaganda fails, a war of national liberation against Fidel Castro must be launched," Tambs was quoted as saying.

A Dec. 5, 1980, press release from Arizona State University quoted Tambs as saying that the United States should cooperate with South Africa in support of national insurgents to "frustrate Castroite adventurism."

Although some conservatives expressed misgivings about Tambs being appointed ambassador to Panama, he retains their support for other jobs. Tambs said yesterday he has been sounded out about being named ambassador to Costa Rica, Argentina or Venezuela.

Howard Phillips, a leading New Right spokesman traveling with Tambs, said the Conservative Caucus, which Phillips heads, plans to launch a 50-state campaign "to promote the restoration of the Monroe Doctrine as the basis for American policy in Latin America."

The controversy over Tambs surfaced as Malcolm Toon, a retired ambassador to Moscow, accused the Reagan administration of appointing unqualified amateurs to important diplomatic posts.

He described the ambassador to Mexico, John Gavin, a longtime friend of Reagan, as "a Hollywood actor, and not a very good one at that."

"We have a man in London who owes his place in life to the fact that his parents founded a furniture polish dynasty. His only qualification for the job is that he speaks English," Toon said in an intervew in the Foreign Service Journal. The ambassador is John J. Louis Jr., a member of the family associated with the Johnson Wax Co.