GENEVA, SEPT. 25 (FRIDAY) -- Sluggish business investment and Third World debt have helped prevent global economic growth from meeting expectations in 1986 and 1987, the General Agreement on Tariffs and Trade said today.

A GATT study concluded that over the past 18 months, the growth-slowing effects of exchange rate fluctuations and a plunge in petroleum prices put a damper on the positive effects of investment, output and trade.

The study predicted that the volume of world merchandise trade would grow 3.5 percent this year, the same as in 1986.

The estimate was revised upward from the 2.5 percent GATT forecast earlier this year.

The study comprises the first chapter of GATT's annual report, due to be published in November.

The 92-nation organization, formed in 1948 to promote international commerce, traditionally releases the first chapter in advance.

The study noted the negative impact of Third World debt on international trade.

The report said Third World debt has lowered the import capacity of indebted countries, which in turn affects the export sectors of other nations.

Moves being made to help the poorest countries are "encouraging," but the report does not support any specific proposal.

"As long as the debt problem remains as severe as it is, it will remain a factor holding back the recovery of the world economy," the report said.

The sharp decline in oil prices was cited as another factor tugging at the world economy.

Spending cutbacks by oil-producing countries had been expected, but "what was not expected was the magnitude of the cutbacks."

In addition, the report said, consumers who found their energy bills reduced failed to spend as much as anticipated on other goods and services.

Exchange rate fluctuations, as expected, dampened export activities in countries whose currencies appreciated since March 1985.

But countries whose currencies depreciated, including the United States, showed only modest increases in export volume or declines in some cases.

The report said it is difficult to judge whether most of the negative effects of the changes in petroleum prices and exchange rates have passed.

It cited the apparent speed with which Japanese industry is adjusting to the stronger yen as one sign that they may have.

However, it said, even if positive factors are now outweighing the negative ones, a boost in the world's economic growth rate "is likely to be modest."

The report also noted:

In 1986, for the first time ever, developing areas earned more foreign exchange from exports of manufactured goods than agricultural or mining exports.

Among the fastest growing product categories in value terms were clothing, motor vehicles, specialized machinery, household appliances, textiles and office and telecommunications equipment.

There continues to be concern over U.S. trade deficits and growing protectionist sentiment.