Pakistan has received a number of illegal shipments, via Canada, of U.S.-made electronics components that are essential elements in the development of a nuclear weapon, according to Canadian and American sources.

Shipment of the equipment, which took place during a period of months earlier this year, wa revealed last week following Canadian acknowledgment that three men has been charged with violating export laws in connection with a whipment of electronics equipment. The shipment, valued at $42,500, was seized at Montreal's Mirabel Airport.

Royal Canadian Mounted Police officials involved involved in the investigation said in telephone interviews that they have evidence of at least five other shipments of similar electronics parts.

The electronics equipment involved is identified officially only as "condensers and resistors," but knowledgeable Canadians say they are components for an inverter, a complex piece of equipment that can be used to enrich uranium to weapons grade or provide electricity needed for the precise manufacture of nuclear weapons parts.

Possessing the equipment is legal in Canada, but shipping it out of the country is against strict export laws designed to monitor who has control of the materials.

Pakistan has consistently denied that it is attempting to manufacture a nuclear weapon, but a U.S. official said, "There is no doubt at all what this material was to be used for," referring to Pakistan's alleged nuclear program.

Pakistani technicians were reported in September to be assembling a clandestine plutonium reprocessing facility near Rawalpindi that U.S. intelligence experts say could enable the country to test its first atomic bomb two years earlier that previously predicted.

The intelligence experts said the samll reprocessing facility could give Pakistan enough fissionable material to state an initial atomic test in the fall of 1981.

U.S. officials eager to curb the spread of nuclear weapons have been presurring major exporters of nuclear equipment not to sell to countries that have refused to sign the Nuclear Nonproliferation Treaty, something neither India, which already has a nuclear "explosive device," nor Pakistan has done. i

A major dispute developed earlier thgis year between the United States and Switzerland about Swiss sales of sophisticated nuclear technology to Pakistan, and there have been a number of reports of Pakistani purchases of equipment throughtout Western Europe, as indication of the tenacity and ingenuity with which the Pakistanis have pursued their appparent goal.

Canadian officials said they began their investigation into the shipments after being informed by their "security services" of the operation. Other sources say the initial warnings came from U.S. and British intelligence.

Disclosure of the investigation also has the potential to embarrass the government of Prime Minister Pierre Trudeau. Canadians are extremely sensitive to any suggestion of carelessness regarding the proliferation of nuclear weapons since a nuclear power plant built by Canada in India was used in the production of a nuclear device exploded in 1974.

Canada provided assistance in the construction of a similar power plant in Pakistan in the 1960s, but ended nuclear cooperation in 1976 after Pakistan refused to meet safeguards intended to ensure that materials from the nuclear plant were used only for peaceful purposes.

Spokesment for the opposition Progressive Conservative and New Democratic parties already have demanded that Canada take up the issue of the shipments with the Pakistanis. The spokesmen were reacting to reports that two Pakistani government officials came to Canada last summer with the alleged intention of coordinating purchases of electronic equipment.

The investigation was made public last week in a current affairs program of the Canadian Broadcasting Corp., which said two officials of the Pakistan Atomic Energy Commission obtained visas earlier this year to come to Canada to work at the Pakistan Consulate in Montreal. But the Cbs said the officials, who were in Montreal from July 7 to 21, never visited the consulate.

The three men facing the initial charges of exporting U.S. goods from Canada without a permit are Salam Elmenyawi, 31, Mohammed Ahmad, 44, and Abdul Aziz Khan, 40. The accused are scheduled to appear in court Dec. 15.

The three men are Canadian citizens. Khan is an engineer originally from Pakistan, Elmenyawi is a businessman originally from Egypt and Ahmad is a mechanical specialist from India. Two Montreal companies were also charged. m

It is not known if the same men or companies are involved in the previous shipments about which charges have not yet been made public.