The accident at the Chernobyl nuclear power plant in April was caused by "a series of gross breaches" of regulations by the station staff, and criminal proceedings have begun against those responsible, according to an official account published today.

Four ranking Soviet ministerial officials have been fired for "bad errors and shortcomings" that "led to an accident with grave consequences," the official news agency Tass said in a dispatch on a special meeting of the ruling Politburo today where a report of a commission that investigated the Chernobyl accident was discussed.

The Tass report, which provided the first official explanation of the causes of the accident as well as the first overall estimate of the financial loss to the Soviet Union, raised the death toll from the previously reported 26 to 28 and estimated the direct cost at $2.8 billion.

It named those who have been relieved of their duties because of the accident, as including Yevgeny Kulov, the head of the atomic power safety committee, and Ivan Yemelyanov, the deputy head of an institute that designs nuclear reactors who two months ago gave interviews to reporters. Kulov's firing was reported yesterday but without explanation.

Today's report singled out the Ministry of Power Engineering and Electrification and the state atomic safety committee for "irresponsibility, negligence and indiscipline" in failing to enforce safety regulations at Chernobyl. It said members of the power station's staff were conducting unauthorized experiments at the time of the explosion.

Anatoli Maiorets, the electrification minister, "deserved to be relieved of his duties for serious shortcomings in the supervision of the Chernobyl station," Tass said. But he was given a reprieve because he had been in the office only since March 1985. He was one of the first of Soviet leader Mikhail Gorbachev's major appointments.

Still, the minister was "strictly reprimanded as a party member" by the Politburo, which warned him of stricter penalties "if he failed to draw the lesson," Tass said. It also reported that a new Ministry of Atomic Power Engineering had been created.

Tass said the Politburo today discussed a report on Chernobyl prepared by a special government commmission, which was appointed after the April 26 incident, the world's most costly nuclear accident.

Direct losses caused by the explosion at one of Chernobyl's four reactors and the subsequent massive leak of radiation are estimated at 2 billion rubles (about $2.8 billion), Tass said. It added, "There are difficulties with power supply to the national economy."

The Chernobyl death toll has been raised now to 28, two more than the last official report, with 203 people suffering from radiation sickness, of whom 30 are still in the hospital.

Tass said people evacuated from the Chernobyl area were being compensated for their losses. It also noted that as a result of decontamination efforts, radiations levels in the area have been "drastically reduced."

The report said an area of about 400 square miles has been contaminated around the station.

"Farmland has been taken out of agricultural production, and work at plants, contruction projects and other organizations has been suspended," it said.

Tass said the investigating commission's report established that at the time of the accident, experiments with "turbogenerator operation regimes" were being conducted while reactor No. 4 was "sidelined for planned repairs at night."

"The managers and specialists of the atomic power station themselves had not prepared for that experiment, nor agreed on it with appropriate organizations, although it had been their duty to do so," the report said.

Previous reports had noted that reactor No. 4 was in the process of being shut down and was operating at less than 6 percent capacity when it experienced a sudden surge of power. The increase in power, described before by Soviet officials, created the conditions for a hydrogen explosion.

Today's Tass report gave no further explanation of the experiments. It said the Soviet Union's chief prosecutor was conducting an investigation, which will lead to court proceedings.

Soviet newspapers previously have reported firings and punishments of local officials in Chernobyl.

Today's report takes the blame up the chain of command to Moscow in this highly centralized system.