The Soviet government, in a move reflecting deep concern about possible acquisition by Peking of advanced Western weapons, has warned Britain against selling Harrier jet fighters to China.

The warning was contained in a letter Soviet President Leonid Brezhnev sent Monday to British Prime Minister James Callaghan. According to British officials quoted in dispatches from London, Brezhnev's letter spoke of serious but unspecified "consequences" involved in sales of British military equipment to China.

Callaghan's answer was expected in the next few days. But he is not likely to yield to Soviet pressure as he publicly stated Tuesday that his government "would not allow our relations with any country to be dictated by a third party."

China is reported to want to purchase about 90 Harrier vertical takeoff fighters in a multimillion-dollar deal. Each plane costs about $8 million.

The deal would have to be approved by a committee proposed of representatives of NATO countries and Japan that reviews sales of advanced Western technology to communist countries. No objections were expected from the Paris-based unit known as Cocom.

The Russians cautioned the West last summer against supplying weapons to China, arguing that the peking government was a "serious threat" to peace since it is involved in "direct expansionist actions."

With the emergence of China's active new foreign policy aimed at creating a "broad united front" against the Soviet Union, the possibility of Western arms shipments to China would increase the danger on Moscow's eastern flank.

Moreover, such moves could conceivably tilt the strategic and political balance against Moscow, fueling the Kremlin's worst fears of having hostile and powerful forces with links to each other on both flanks.

Chinese military officials visited a number of NATO countries earlier this year looking for Western weapons. Later Peking completed an agreement for French antitank missiles and expressed interest in British aircraft and the West German Leopard tanks as well as various Western satellite systems.

The Harrier jet, which is used by U.S. forces, is viewed as well suited for Chinese purposes. It needs no air base for operations and could be successfully deployed in the rugged terrain of northern China along the Soviet border. It can land on a variety of surfaces such as grass, tarmac, concrete, dirt and gravel strips, snow and ice-covered runways.

The aircraft is the West's only operational fixed-wing vertical takeoff and landing strike fighter. It also can be operated from ship decks.

China's Vice Premier Wang Chen reiterated Peking's enthusiasm for the aircraft during his visit to Britain earlier this month. During Wang's visit the two countries drew up a plan for massive expansion of bilateral trade with the aim of reaching the annual figure of $10 billion by 1985.

British sources were quoted as saying that similar Soviet warnings against sales of arms to China were delivered to other Western leaders.

The Harrier sale itself could hardly tilt the current Sino-Soviet strategic balance, which is overwhelmingly in Moscow's favor. But the deal estimated to total about $700 million would provide a boost to the ailing British aircraft industry.

Brezhnev's strong letter to Callaghan coincides with Soviet diplomatic pressure to halt the Harrier deal.

"Soviet diplomats are doing all the arm-twisting they can decently get away with," Reuter quoted a British official in London as saying.

Apart from Peking's growing ties with NATO countries, the conclusion of a Sino-Japanese friendship pact pledging joint efforts to oppose any third country's efforts to establish "hegemony" in the Asia-Pacific region has heightened Soviet anxieties about possible encirclement.

The Russians have argued that Peking's warmth toward the West is tactical and that the new Chinese leaders are seeking to damage Soviet-American relations and ultimately provoke a confrontation between the two superpowers.