LONDON (AP) _ The most common nuclear reactors in the Western world are developing mysterious cracks that strongly increase the risk of a reactor meltdown, according to a Greenpeace report released Friday.

Pressurized water reactors built by American, French and Japanese companies are at highest risk, the environmental group said.

Antony Frogatt, a nuclear campaigner at Greenpeace International, said, ''The meltdown potential of Western reactors has risen alarmingly. Immediate shutdown of cracked reactors is a clear necessity, and all reactors at risk have to be comprehensively inspected right away.''

Greenpeace accused power companies and safety authorities responsible for the reactors of not taking the threat seriously.

''Inspection programs are haphazard and unsystematic, and in the country with the most to fear - the United States - have not begun at all,'' it said.

Joe Fouchard, spokesman for the U.S. Nuclear Regulatory Commission in Washington, said the cracking ''has not become a problem here ... But we are developing a program with American utilities to inspect for these cracks.''

Carl Goldstein, a spokesman for the U.S. Council for Energy Awareness, the communications arm of the nuclear industry, said the cracking ''is not a catastrophic phenomenon. ... It is a preoccupation and we are looking at it.

''It's unfortunate that Greenpeace, in order to be heard, has to talk in apocalyptic terms.''

The first cracks were discovered in September 1991 during a routine test on a French 900-megawatt pressurized water reactor. They were discovered in the huge pressurized container that holds the burning nuclear core.

Since then, cracks have been detected in 13 out of 18 French reactors which have undergone inspections. Cracks also were found in all three pressure water reactors in Sweden, one in Switzerland and one in Belgium, the report said.

Frogatt said the cracking is an example of ''premature aging of nuclear reactors.''

''The key components are wearing out, which may well lead to their functioning incorrectly at a key time. Often these failures cannot be detected.''

The cracks are occurring in tubes at the top of the pressurized container that carry the control rods into the heart of the nuclear core. Control rods are the basic safety component of nuclear power plants because they can stop the nuclear chain reaction.

If enough control rods could not enter into the reactor core in an emergency, operators could not immediately stop the nuclear reaction, which could lead to a meltdown and the release of dangerous radiation.

The reactors at highest risk of developing the cracks appear to be those which used a nickel alloy called Inconel-600 in the control rod tubes, Greenpeace said.

These reactors were built by the American companies Westinghouse, Babcock & Wilcox, and Combustion Engineering; Mitsubishi Heavy Industries of Japan, and the French firm Framatome.

There are currently 188 pressurized water reactors operating in the United States, France, Japan, Spain, Belgium, Sweden, Switzerland, Taiwan, South Korea, Germany, Brazil and South Africa.

Greenpeace said it did not know how many used Inconel-600.

Frogatt said nuclear experts first suspected stress was causing the cracking, but now realize the problem is far more complex and do not know why it is happening or whether Inconel-600 is the sole cause.

The first cracks were lengthwise, and for more than a year nuclear authorities denied the problem could lead to a major disaster because they believed such cracks would leak before they actually ruptured, the report said.

But it said that in December, laboratory testing of the control rod mechanism from France's Bugey-3 reactor revealed cracking around the tubes, and those cracks could rupture suddenly, without leaking.

The Greenpeace report was written by Helmut Hirsch and Thomas Panten, nuclear engineers with Gruppe Oekologie in Hannover, Germany; Norbert Meyer and Detlef Rieck, nuclear engineers formerly working in nuclear power plants in East Germany; and Mycle Schneider, of the World Information Service on Energy in Paris.