STOCKHOLM, Sweden (AP) _ Rumors of another Soviet nuclear accident swept across Western Europe on Wednesday, triggering speculation in grain and dollars on financial markets before being squelched by Soviet and Swedish officials.

The false reports may have been the fallout from a test of an early warning system begun by an international nuclear energy watchdog agency in the wake of the 1986 Chernobyl disaster in the Soviet Union.

Wednesday's rumors apparently began in Sweden, as did the first reports of the accident at the Chernobyl nuclear power plant in the Soviet Ukraine.

The national news agency Tidningarnas Telegrambyra, citing Sweden's national Radiation Protection Agency, reported ''an atomic power plant accident apparently occurred in the Soviet Union.''

The report spread quickly, even though officials at the radiation agency said they had made no such statement and the Soviet Minister of Nuclear Energy denied an accident had occurred.

The rumor also reached New York, as stock prices of companies that trade in grain rose on the rumor, because contaminated Soviet crops would spur U.S. exports.

''The dollar could go above 1.70 (West German) marks if the rumor of the nuclear accident proves to be true,'' said Hubert Pedroli, vice president at Credit Suisse bank in New York.

The dollar, a traditional money shelter in troubled times, rose slightly on some European markets as the rumor spread. ''Everyone started buying dollars like crazy,'' said a trader at a large U.S. bank in Frankfurt.

It ended the day mostly lower as denials about an accident poured in.

The Swedish radiation agency said no increased levels of radiation were reported from anywhere in Sweden. The U.S. State Department also said it had received no reports of increased radiation.

The official Soviet news agency Tass took the unusual step of issuing the following denial: ''A number of foreign news agencies today circulated stories about an alleged nuclear accident in the U.S.S.R. A Tass correspondent inquired at the U.S.S.R. Ministry of Atomic Power Generation and was officially told that there has been no accident at atomic power stations in the U.S.S.R. All atomic power stations in the U.S.S.R. are functioning normally and all stories to the contrary are groundless, the Tass correspondent was told.''

The Soviet Union came under international criticism when it delayed reporting the Chernobyl accident for several days as a cloud of radioactivity spread across Europe. That accident, the worst in the history of nuclear power, caused 31 deaths.

The International Atomic Energy Agency in Vienna, charged with running the accident communication system in the wake of the Chernobyl disaster, was among the first to contradict Wednesday's reports. However, there were indications the agency may have inadvertenly fueled the rumor by running tests of its own system.

Authorities in Britain, Norway and Washington said the scare may have been caused by messages transmitted over open telex lines in the past week from the Vienna agency to test the facilities of the World Meteorological Organization, based in Geneva, Switzerland.

The alert system is part of an agreement signed by the Soviet Union and other agency member states following the Chernobyl disaster to quickly warn each other in case of an accident.

''The messages may have been misunderstood by outsiders and thus caused groundless rumors,'' said Per Paust, a spokesman for the Norwegian Foreign Ministry.

IAEA spokesman Hans-Friedrich Meyer said the agency had sent a series of ''test telexes'' to the Geneva organization to make sure the early warning system would work in an emergency, Meyer said.

The last such telex was sent Jan. 27, he said. Jenny Stickings, a spokeswoman for the meteorological organization, confirmed the date and said it involved 20 to 25 different countries.

The various meteorological services involved were asked to send confirmation they received the test message to the IAEA and to the Geneva organization, she said. Somebody ''not in the know'' about the test could have seen a reply message and thought it referred to a real nuclear accident, she added.

The meteorological organization is investigating the incident.

The nuclear watchdog agency denied culpability. ''At no time has the agency used the word 'accident' in these tests,'' said an agency statement.

TT, in a report of its handling of the story, said it had heard the rumor and called the Swedish radiation agency, where an unidentified source confirmed that an accident had happened.

Editors at TT declined to identify the source or the reporter.

Other reports on the rumor cited sources as various as the Finnish power board and London currency exchanges.

There also was a chemical spill following a train derailment in the Soviet Union on Monday, and ''maybe somebody got confused,'' said Gunilla Ericsson, a radiation protection officer at the Swedish radiation agency.

Tass reported Tuesday that a train derailed in Jaroslavl, northeast of Moscow, spilling a toxic chemical that caused the temporary evacuation of 3,000 people. The agency said 34 people were hospitalized with signs of slight poisoning.