Six prominent leaders in Europe and the developing world told President Reagan in a letter released yesterday that they are "deeply disappointed" by the recent U.S. underground nuclear test that brought an end to the Soviet Union's self-imposed testing moratorium.

The leaders, who have been active in proposing nuclear test bans and suggesting improved monitoring of them, also asked Reagan and Soviet leader Mikhail Gorbachev in separate letters to hold another summit meeting "at the earliest possible time" and to refrain from further nuclear testing until then.

The question of nuclear testing is "too important for us, and for all the peoples in the world, to be left only to the nuclear weapons states," said the letters to Reagan and Gorbachev from President Raul Alfonsin of Argentina, Prime Minister Rajiv Gandhi of India, President Miguel de la Madrid of Mexico, Prime Minister Andreas Papandreou of Greece, former president Julius Nyerere of Tanzania and Prime Minister Ingvar Carlsson of Sweden.

Carlsoon replaced Olaf Palme, who was assassinated Feb. 28, as a member of the group, known as the Five Continent Peace Initiative.

The six leaders also told Reagan and Gorbachev that continued development of nuclear weapons by states possessing them "would be detrimental to the efforts to prevent any further proliferation of nuclear arms to other countries."

Two of the six nations, India and Argentina, are among those considered on the threshold of a major nuclear weapons capability.

"We are convinced that it is possible to adequately verify compliance with any halt in nuclear testing," since both superpowers have declared their willingness to accept on-site inspection, the letters to Reagan and Gorbachev said.

Soviet Foreign Minister Eduard Shevardnadze said Friday in Moscow that his country would permit seismic devices provided by the six world leaders around Soviet nuclear testing sites in the context of a full-scale moratorium or ban on future testing, according to Nicholas Dunlop, secretary general of Parliamentarians Global Action, which has provided staffing for the six leaders.

Dunlop, who participated with others in the meeting with Shevardnadze, quoted a lower-level Soviet Foreign Ministry participant as saying that technicians from outside would be welcome to monitor compliance with a test ban.

Soviet participants also said that the idea of an international verification agency to monitor test bans is "very promising" and "worth exploring," according to Dunlop.