The United Auto Workers Union petition to cut the number of imported cars and trucks entering the country in half could heighten already tense world economic relations, an European Economic Community spokesman said yesterday.

The union petition to cut imports, mainly those from Japan and impose drastically higher duties on imported cars and trucks probably wouldn't result in retaliation by the Europeans, but "It would just worsen the situation more," the EEC spokesman said. The UAW filed the petition Thursday.

The UAW action coupled with anti-dumping complaints filed earlier this year against steel producers in seven ECC countries is "just one more issue that's creating a difficult atmosphere in economic relations," he said.

Meanwhile, General Motors Corp. Chairman Thomas A. Murphy told reporters yesterday he opposes government quotas to cut the number of autos imported into the U.S. But if the Japanese had exhibited "a more enlightened attitude" to ease the import problem the autoworkers wouldn't feel pressured to petition the government for import relief.

"I don't think this country should take a protectionist stance," Murphy said. "We would rather compete in the marketplace and convince Americans to buy U.S.-made cars."

The UAW, with 300,000 of its members out of work, asked the International Trade Commission to cut imports to levels reached in 1975 or 1976. In addition, the union asked the ITC to raise duties on imported cars from 2.9 percent to 20 percent and maintain the 25 percent import duty on light trucks.