The Canadian government yesterday warned that an effort to establish trade "reciprocity" would seriously impede multilateral trade negotiations.

Allan E. Gotlieb , ambassador of Canada to the United States, told a New York audience that while the reciprocity notion at first blush seems a fair response to protectionist tactics, it would actually prove to be inflexible and, in the end, "is simply not achievable in the real world."

He said that to deal with protectionism, "the best defense is a good offense," and recommended as an alternative to reciprocity an effort to "reestablish the momentum for further trade liberalization."

The notion of "reciprocity" has gained currency lately as a consequence of huge trade imbalances between major trading nations, notably the surpluses that Japan has consistently recorded in its trade with Europe and the United States. The rationale for the idea is that it assures "fair trade" when "free trade" is not possible. Under a reciprocity rule, an importing nation would allow foreign goods access to its domestic markets only on the same terms and conditions as its goods are permitted access to the exporting nation's markets.

Recently, Commerce Secretary Malcolm Baldrige--speaking for himself--said he would support a current congressional groundswell for reciprocity unless the Japanese opened their markets to American products, including high-technology items for which the Japanese are giving American companies a strong run for the money here at home.

But the Reagan administration has not taken a formal position on the question of reciprocity.

But Gotlieb, in a speech billed by the Canadians as the first official view from Ottawa on the reciprocity subject, urged caution in a speech to the American Association of Exporters and Importers.

If the reciprocity movement gains enough momentum, Gotlieb said that everybody would be forced to negotiate special bilateral deals, "and it then would not be long before we recreated the 'beggar-thy-neighbor' approach of the Great Depression. And, one thing is certain: if international trade shrinks, we shall all be losers."