Failure of the Bonn economic summit to set a starting date for global trade negotiations is likely to increase protectionist pressures in Congress, Secretary of Labor William E. Brock said yesterday.

Brock, who until last week was U.S. trade representative and had worked for more than a year to get agreement on new talks, said in an interview that if the preparatory meetings now scheduled for July should bog down, "there will be a lot of pressure on Congress to take unilateral action."

Beyond the specific trade issue, there was a sense of dismay in Washington on the more generalized lack of results at the Bonn economic summit, especially at a time when the U.S. economy -- the main force in global growth during the past two years -- may be entering a slack period.

The existence of an actual starting date for a new trade round, Reagan's declared objective, could have deflected specific protectionist bills, Brock said. But so long as the administration is not able to cite an imminent starting date, he added, negative trade news, such as high monthly trade deficits, will stir emotions on Capitol Hill.

U.S. officials were disappointed, but not surprised, by the inability to get unanimous agreement in Bonn, even though at the end, six of the seven countries wanted to insert 1986 as the starting date. French President Francois Mitterrand had indicated earlier that he would attempt to delay setting of a new trade round.

The officials said that Mitterrand, while publicly linking a demand for thoroughgoing monetary reform with trade negotiations, actually fears a trade round that might subject France to new competition in agricultural and high-technology markets.

"The nub of it is that this is the last Maginot Line of defense against doing anything about the Common Agricultural Policy," said an administration official. That policy establishes minimum prices for European farm products. The essence of the American charge is that Mitterrand, who faces reelection in 1988, fears that in negotiations, some of the highly protective floor prices for French farm products might have to be adjusted downward.

The same source said that Mitterrand appears equally worried that France may not be competitive in the services and high-technology products that the United States hopes to put on the table in negotiations. It was for the same reason, American officials said, that Mitterrand rejected Reagan's proposed research program for the space-based missile defense system commonly referred to as "Star Wars."

Administration officials are still hopeful that a trade round can be started in 1986. But other trade experts said Mitterrand's stance inevitably delays that prospect and does not bode well for the prospects of an effective round, should it ever get started.

The administration's broad hopes for the summit were keyed to a multifaceted package deal at Bonn. In addition to the new trade round, Reagan went to Bonn committed to get Western Europe and Japan to expand their economies to replace some of the declining thrust of the American economy.

The American contribution to this package was to have been a strong pledge to reduce the $200 billion U.S. budget deficit, leading to lower interest rates and a more stable dollar, and a new willingness to consider international monetary reforms.

But as former West German chancellor Helmut Schmidt said yesterday on ABC television, each country instead offered to continue to do, more or less, what it is doing at the moment.

With some bitterness, the U.S. team lay the blame almost wholly on Mitterrand. "French politicians believe they gain when they're isolated," a high U.S. official had said at a previous meeting. "They like to say to the French people, 'If we're isolated, we must be right.' "

A few days before the summit, Jean-Daniel Tordjman, economic minister at the French Embassy in Washington, asserted that "there is no major trade problem. There are some specific issues in the world, but no major trade problem. Global trade, world trade is in quite good shape. So what is the real problem today? The real problem today is the erratic variation of the currencies. We need more consultation between our countries."

But all of the other summit countries disagreed. They saw a new round as critical for spurring economic growth and curbing protectionist pressures. Secretary of State George P. Shultz said on the eve of the summit that it was protectionism that had led to the Great Depression. He suggested that the issue was so important that the United States might have to pursue trade liberalization with other countries on a bilateral basis if the Bonn summit refused to get a start on multilateral talks.