U.S. Undersecretary of State James L. Buckley plans to return to Europe after his current trip with a "suggested formula" for tightening the West's credit policies toward the Soviet Bloc, a senior American official said today.

The formula, to be worked out in Washington, would be designed to avoid "trade disadvantages" among Western countries competing for business with Moscow and its satellites, the official said.

The Reagan administration reportedly favors curbs on loans to the East to create difficulties for the Soviets, who are suffering from shortages of hard currency to pay for Western technology and grain.

Buckley left for London--the next leg of his five-nation tour--after conferring with French Foreign Minister Claude Cheysson, Defense Minister Charles Hernu and officials at the Organization for Economic Cooperation and Development.

The Buckley mission, designed to seek European backing for tougher sanctions against the Soviets for their support of the military takeover in Poland, was still pressing U.S. allies to block or delay implementation of the Soviet gas pipeline deal, according to the U.S. official.

He indicated that, despite reports to the contrary, the Reagan administration had yet to reconcile itself to the West European plan to buy billions of dollars of Soviet gas that would be carried from Siberia by the 3,500-mile pipeline.

But reflecting both West German and French determiniation to push ahead with the controversial project, French Foreign Ministry sources said only that officials had "listened" to Buckley and his delegation.

Europeans have questioned what the Reagan administration could offer them in return for forgoing the pipeline deal. The senior American official acknowledged that French officials brought up with Buckley continued U.S. grain sales to the Soviets. In the European view, the sales tend to weaken American arguments against the pipeline.

The official said the U.S. government felt it was on firm legal ground if it chose to prevent the already negotiated sale to the Soviets of rotor blades for the pipeline designed by General Electric and licensed for manufacture here by Alsthom Atlantique.

Plans for a second trip by Buckley appeared to undercut theories that he had delayed his European trip so long that his presence here amounted to little more than a gesture to hard-line American domestic opinion pressing for harsher anti-Soviet sanctions.

On other matters, the American official said last Friday's meeting in the White House between President Reagan and French President Francois Mitterrand had concentrated on Central America.

The official suggested that Reagan had dissuaded Mitterrand from pursuing closer links with Cuba, and specifically from inviting Cuban leader Fidel Castro to visit Paris. A Castro visit had been suggested by some members of Mitterrand's Socialist Party.

Buckley's trip began in Bonn and will take him to Rome and Brussels after London.