A United Nations study has concluded that the world tobacco industry is dominated by an "oligopoly" of seven multinational conglomerates that erect "virtually" insurmountable barriers" to potential competitors.

The companies' control of global markets is maintained in part by expenditures of at least $1.8 billion to advertise existing brands and promote dozens of new ones, the study says.

In developing countries, where per capita tobacco use is rising faster than in industrial nations, "contstraints" on cigarette advertising tend to be less severe, the report goes on.

Developing countries now supply about 55 percent of world leaf tobacco exports. However, these countries' exports of processed cigarettes are "almost nonexistent" and "their aggregate receipts from the tobacco industry are based, almost exclusively, on the demand and marketing decisions [of] transnational tobacco corporations."

In the world tobacco industry, "a small number of giant buyers is matched by a small number of giant sellers," the report says.

The 122-page report was prepared by the secretariat of the U.N. Conference on Trade and Development (UNCTAD) and released recently after what one U.N. source described as "months of bureaucratic delay."

A vigorous protest about the report was reportedly lodged with UNCTAD by one major company, and British government representatives also were said to have raised questions about it.

A spokesman for the leading U.S. tobacco company, R.J. Reynolds of Winston-Salem, N.C., said the report was "so biased and inaccurate that we don't think it's worth any detailed comment."

Sen. Robert Morgan (D-N.C.) asserted in a statement that "this is one of the most astonishing documents ever released by a government agency at any level. It is astonishing for its sensationalism, selectivity and reckless disregard for balance."

The study is one in a series produced by UNCTAD under a U.N. General Assembly mandate.

An earlier study was critical of the role of multinational companies in the world banana economy.

A study prepared for UNCTAD last year by economics Prof. Richard S. Newfarmer of the University of Notre Dame detailed the domination of the world electrical industry by a "small group of very large, highly diversified enterprises" that have capture "spheres of influence" in various geographic regions of the world. The U.N. agency cautioned that Newfarmer's views did not necessarily reflect those of the secretariat, but such a caveat was not attached to the report on tobacco.

The authors of the tobacco study conclude that "unless the [domination] of the tobacco companies is matched by effective countervailing power at the national level, the present play of forces must lead to further concentration, with all its implications."

They list seven firms accounting for $32 billion worth of gross sales per year and 58 percent of all cigarette production in the non-Communist world. They are British American Tobacco and International Tobacco of Britain: R.J. Reynolds, Philip Morris, American Brands and Gulf and Western of the United States, and the Rupert.Rembrandt and Rothman Group, a South African holding company.

In addition, the report says, 85 to 90 percent of the tobacco leaf entering world markets "is under the direct or indirect control of approximately six multinationals," which sell to the worldwide plants of the big cigarette producers.

Three companies dominate the manufacture of high-speed equipment that can turn out up to 5,000 cigarettes a minute. The largest is Molins, which is partly owned by the two leading British cigarette companies.

All of the major companies have channeled earnings into far-reaching diversification programs that have strengthened their economic power. The South Africa group has interests in gold, uranium, coal and other minerals. Philip Morris owns the Miller Brewing Co. Reynolds has branched into shipping, petroleum and aluminum.

According to UNCTAD, advertising, rather than price competition has become the main menas for these large firms to strengthen their domination of cigarette markets. The report says that "world advertising through which the tobacco multinationals work" is concentrated in the hands of 10 international firms that had billings of $7.1 billion in 1975.

"The link between international advertising agencies and the transnational tobacco companies is a conspicuous feature of the tobacco industry, where advertising has become an important barrier to entry [competition from outsiders], as well as a vital means of influencing consumers," the report asserts.

In 1975, 70 percent of the advertising billings of all kinds of developing countries was accounted for by foreign firms.

With the cost of marketing a new cigarette brand ranging from $15 million to $40 million, smaller companies have trouble competing with the majors. UNCTAD said new brands are "proliferating" and help "sustain an abnormally high level of corporation profitability."

Nationally based firms are a target of takeovers in developing countries.

Both Philip Morris and R.J. Reynolds acquired local companies in Brazil in the 1970s, challenging the traditional sphere of influence of British American Tobacco.

In the United States, R.J. Reynolds, Philip Morris and British American's subsidiary, Brown and Williamson,account for 76 percent of the cigarette market.

"The increasing concentration of economic power, opitomized by the transnational tobacco companies, is a crucial element in the world economy," the UNCTAD analysts write.

The report also includes extensive reference to the tobacco industry's growth, early antitrust battles and more recent involvement in corporate payoffs. "In addition to mass advertising, another integral component of world tobacco marketing is global corporate bribery, or the 'payoff complex,' involving millions of dollars," says the study.