President Valery Giscard d'Estaing is planning to meet with Soviet President Leonid Brezhnev somewhere in Poland, probably starting on Sunday, diplomatic sources said here today.

The French president's willingness to be the first Western leader to hold a summit meeting with the Soviets since their invasion of Afghanistan is seen as a major breakthrough for Soviet effforts to punch holes in the already frayed solidarity of the allies.

The U.S. government is reliably understood to have been informed today by the French, but not consulted about the planned meeting in Poland.

Reports from Warsaw said it would take place at Wilanow, the one-time summer palace of the Polish kings a few miles south of the capital. They said that the palace had been closed to tourists through Wednesday.

French and other Western sources said Giscard apparently cannot expect anything beyond what the Soviets have already been saying in previous contacts. The French justification for the meeting is said to have been confined to speaking of the need to continue an East-West dialogue to preserve detente. Officially, France condemns the Soviet invasion, but argues that it should not be allowed to poison the international atmosphere. France was also the first Western power to resume contacts with the Soviets at the foreign ministers' level, with the visit here last month of Andrei Gromyko. French officials described his talks with Foreign Minister Jean Francois-Poncet as being surrounded by a "Siberian chill."

Nevertheless, the French ambassador to Moscow was the only Western envoy to attend the traditional May Day parade in Moscow's Red Square.

The French Olympic Committee voted this week to deffy the U.S. call for a boycott of the Moscow Games this summer and the French government announced that it would provide the financial backing needed by French athletes.

Francois-Ponet met again yesterday with Gromyko after the Vienna ceremonies for the 25th anniversary of Austrian neutrality.

Soviet officials here have been saying that it is also time to resume the Soviet-American dialogue, but a meeting yesterday in Vienna between Gromyko and Edmund Muskie, the new secretary of state, apparently produced no results.

Word of the Giscard-Brezhnev plans came through carefully orchestrated official leaks in Warsaw. Apparently the French president's office was embarrassed. Officials at the Elysee Palace refused to comment on the reports. Foreing Ministry officials said they were given no guidance on what to say.

Privately, several ministry officials said the Elysee had been mousetrapped by the Soviets and Poles, who let the news out before the arrangements had ben completed to French satisfaction.

"This is purely an Elysee operation. The Foreign Ministry had nothing to do with it," said a French diplomat.

News of the Franco-Soviet meeting came just as representatives of 38 Islamic countries were gathering in Islamabad, Pakistan, to renew their condemnation of the Soviet invasion. The French are hopeful of maintaining their current prestige among Third World countries. The Islamic conference's denunciation of the invasion in January was widely viewed as a major setback for Soviet influence in the Third World. Word of a French meeting with Brezhnev could serve to tone down the Islamic stand this weekend.

The Giscard visit to Poland also makes it far easier for West German Chancellor Helmut Schmidt to accept the pressing Soviet invitation to go to Moscow to discuss the future of detente in Europe.

Schmidt and Giscard have a close relationship and it has been enhanced since the Soviet move into Afghanistan. They have shielded each other from pressures of the Carter administration -- which has complained about lack of allied solidarity.

Schmidt and Giscard, like President Carter, are deeply involved in reelection efforts. Preserving West German's tight network of economic, political and family ties with Eastern Europe is considered essential for Schmidt's reelection. Giscard needs the tacit complicity of the large French Communist Party to defeat Socialist leader Francois Mitterrand in the French presidential elections next spring.

The Giscard-Brezhnev meeting apparently was arranged by Giscard's personal troubleshooter, former interior minister Michel Poniatowski. He went to Poland April 25 to deliver a personal message from Giscard to Polish Communist Party chief Edward Gierek, according to the Polish news agency. It was the same day that Gromyko was ending his chilly talks in Paris.

A highly reliable source said Poniatowski flew to Poland in a plane owned by Jean-Baptiste Doumeng, France's "Communist millionaire," who handles the bulk of France's lucrative farm exports to the Soviet Bloc.

The Poniatowski-Doumeng link is considered to be Giscard's main channel of communications with the French Communists.

Relations between Poniatowski and Foreign Minister Francois-Poncet are known to be tense. Poniatowski regularly makes sharp, often anti-American statements that are regarded as representing aspects of Giscard's real thinking not officially expressed.

This has complicated the task of French diplomacy on several recent occasions. It tends to undermine the credibility of the Foreign Minister's relatively hard line against current Soviet policy.

Giscard and Brezhnev last met a year ago when the French president visited the Soviet Union for the annual summit that is provided for under a Franco-Soviet friendship agreement. It was Brezhnev's turn to come to Paris this year, but he was not expected before fall.