TORONTO, OCT. 31 -- The Canadian government today issued an order blocking an attempt by the U.S. Congress to extend a longstanding trade embargo against Cuba to U.S. subsidiaries in Canada.

Joe Clark, Canada's secretary of state for external affairs, said the trade proposal, which was passed just before Congress recessed over the weekend, is "clearly unacceptable" and would have "a negative impact on trade which Canadian subsidiaries have built up with Cuba over the years."

The measure is awaiting President Bush's signature to become law.

Canada not only maintains diplomatic and trade relations with Cuba, but it also has set itself the uphill task of trying to thaw relations between Washington and Havana since the Cuban revolution three decades ago.

Justice Minister Kim Campbell, who also signed today's order, said, "This action demonstrates our determination to block measures that infringe Canadian sovereignty. It is the government of Canada which makes the laws and policies that govern international trade and commerce by Canada-based companies."

A spokesman for the Canadian Embassy in Washington said that trade between Cuba and U.S. subsidiaries here amounted to only $22 million last year but that the Canadian government views the U.S. measure as a serious violation of international law and an extraterritorial infringement that cannot go unchallenged.

The trade embargo was sponsored by Sen. Connie Mack III (R-Fla.) as an amendment to the Export Administration Act, which, among other things, would put into law the economic embargo imposed by Bush against Iraq after it invaded Kuwait.

The order by Clark and Campbell was based on Canada's never-used Extraterritorial Measures Act, which was adopted by Parliament in 1984 and empowered Ottawa to force any Canada-based company to ignore orders from a foreign parent company or government if those orders are found to be detrimental to Canada's sovereignty or commercial interests.

The Canadian Embassy official said that since 1963 Canada has resisted attempts by Washington to regulate trade by Canada-based companies with Cuba and that in recent months the government of Prime Minister Brian Mulroney has made it clear to Congress and the Bush administration that such a measure would be unacceptable.

The official noted that there is some uncertainty over whether Bush intends to sign the trade measure, but he said that today's action by Canada's government was intended as "a message to show the U.S. administration that we're serious about it."

What remained unclear tonight was how Canada-based U.S. subsidiaries, threatened with fines and other enforcement measures available to both the U.S. and Canadian governments, would react if Bush signs the bill.

Total Canadian exports of cereals and other foods, pulp and machinery parts to Cuba have fallen in recent years, to $154 million last year from $362 million in 1986. Total imports from Cuba, largely sugar, lobsters and precious metals, were valued at $62 million last year. Those figures represent U.S. subsidiaries here and wholly Canadian firms.