In his speech Wednesday night, President Reagan took the unusual step of revealing an intelligence secret to support his contention that "the Soviet Union . . . deserves a major share of the blame for the developments in Poland."

The secret involved the printing of the martial law decree issued in Poland on Dec. 13. "It is no coincidence," Reagan said in his speech, "that the martial law proclamations . . . were being printed in the Soviet Union in September."

Most of official Washington took yesterday off, and those few officials who could be reached declined to elaborate. One senior official said it came from "sensitive but highly reliable intelligence sources. We were assured it was firm stuff." This source added that it was a vivid example of Soviet foreknowledge of the crackdown in Poland.

Presumably the information came from inside the Soviet Union or Poland. One retired Central Intelligence Agency official, Harry Rositzke, said it was much more likely that the source was Polish.

Rositzke and other experts agreed that Reagan's remark was an unusual breach of official secrecy and also a strong message to the Soviets--"if it's true." He said that Solidarity union spokesmen have been saying things of questionable accuracy since martial law was imposed.

A State Department official suggested that it might have been necessary to print the martial law decree in the Soviet Union to avoid a leak to Solidarity in Poland.