An Italian judge sent shockwaves through the scientific world on Monday when he sentenced seven of the country's leading experts on natural disasters to six years each for giving false assurances before the earthquake that hit the city of L'Aquila in 2009.

More than 300 people died after a 6.3-magnitude tremor hit the central Abruzzo region. The earthquake wrecked L'Aquila's historic centre, injured more than 1,000 people and left tens of thousands homeless.

The seven defendants, who belonged to the National Commission for the Forecast and Prevention of Major Risks, were accused of offering an unjustifiably optimistic assessment to the local population a week before the disaster. By then, the area had been hit by some 400 tremors over a period of four months and a local researcher had warned of the risk of a major earthquake, largely on the basis of abnormal radon emissions.

But after an extraordinary meeting of the commission in L'Aquila, one of the experts told a press conference that the situation was "normal" and even "favourable" because potentially destructive energy was being released through the tremors. The prosecution, which brought charges of multiple manslaughter, maintained that lives could have been saved had people not been persuaded by the assurances to remain in the area.

The sentences handed out by judge Marco Billi were higher than those demanded by the prosecution, which had asked for the accused to be given four years each. The judge also imposed lifetime bans from holding public office and ordered the defendants to pay compensation of €7.8m (£6.4m).

Marcello Petrelli, a lawyer for one of the experts, called the outcome of the trial "astounding and incomprehensible". In Italy, convictions are not considered definitive until after an appeal, so it is unlikely that any of the defendants will go to jail immediately.

But the sentences are expected to cause uproar among scientists worldwide. Several international bodies had warned that a guilty verdict could deter scientists from advising governments in future.

Enzo Boschi, the former president of Italy's National Institute for Geophysics and Volcanology, said he was "dejected and in despair". He said he had been convinced that he would be acquitted, "because I have never reassured anyone. I defy anyone to find in writing or speech, on television or elsewhere a reassurance by me concerning the Aquila earthquake."

Luciano Maiani, the incumbent president of the Major Risks Commission, said the verdict marked "the death of the services provided to the state by professors and professionals. It is impossible to supply the state with advice in a professional and composed way under this crazy judicial and media pressure. This does not happen in any other country in the world."

Giampaolo Giuliani, the researcher who became the "Cassandra" of the disaster after his warnings were ignored, said he had expected lighter sentences.

"I do not derive any pleasure [from the outcome]", Giuliani said. "No conviction can repay us for what happened."

• This article was amended on 23 October 2012. The original headline stated that the scientists had already been jailed. This has been corrected.