And they said the quantity of the deadly substance found on his remains pointed to the involvement of a third party.
"We can't say that polonium was the source of his death... nor can we rule it out," said Professor Francois Bochud of the Lausanne Institute of Applied Radiophysics.
Bochud's lab, which is part of Switzerland's respected Vaudois University Hospital Centre, measured levels of polonium up to 20 times higher than it is used to detecting.
'Third party involvement'
"By definition, that indicates third party involvement," he said.
"Our results offer moderate backing for the theory of poisoning."
Arafat died in France on November 11, 2004 at the age of 75 after falling sick a month earlier, but doctors were unable to specify the cause of death.
In November 2012 his remains were exhumed and samples taken, partly to investigate whether he had been poisoned -- a suspicion that grew after the assassination of Russian ex-spy and Kremlin critic Alexander Litvinenko in London in 2006.
Bochud's colleague Patrice Mangin said the lab could have been categorical in its findings if it had been able to obtain biological samples immediately after Arafat's death.
But no post-mortem was performed at the time, at the request of his widow.
The Swiss lab results were first published on Wednesday by the Qatar-based satellite channel, Al-Jazeera.
Since Arafat's death, Palestinian society has long given currency to the rumour that he was murdered, with Israel the party most often blamed.
But there has never been any proof.
With the publication of the Swiss report, Palestinian officials said there was no longer any doubt, and demanded a formal international inquiry into who was responsible.
"The results prove Arafat was poisoned by polonium," said senior PLO official Wasel Abu Yusef.
"This substance is owned by states, not people, meaning that the crime was committed by a state," he told AFP calling for an "international committee" to probe the killing along the lines of the one that investigated the murder of Lebanon's Rafiq Hariri.
"From the day of Arafat's death, the Palestinian street knew Arafat was poisoned to death, but no one knew what the substance was," political analyst Hani al-Masri told AFP.
"Palestinians are now expecting their leadership to follow up on these results."
Speaking to Euronews on Thursday, Arafat's widow Suha said the poison must have been administered to her husband by a traitor in his entourage.
"This must have been put in his tea or in his coffee or in his water. Somebody who was near him must have given him this," she said.
"It's so difficult to doubt, but unfortunately it's somebody (from) his entourage."
Other senior Palestinian officials agreed that the limited availability of polonium-210 strongly suggested the involvement of a foreign government.
"The means used and the fact it was polonium -- which only states use -- suggest that it was a decision taken at a high level in one of those countries," senior PLO executive Ahmed Majdalani told AFP.
But Israel has repeatedly denied any involvement in Arafat's death, with its foreign ministry describing the allegations as a long-running "soap opera."
A top aide to former Israeli prime minister Ariel Sharon, who was in power when Arafat died, told AFP the premier had ordered that no harm be done to the Palestinian leader.
"Ariel Sharon ordered that everything be done to ensure that Arafat, who was at the time living inside his besieged Muqataa compound, was not killed by our soldiers," Raanan Gissin told AFP.
"His instructions were to take every precaution to avoid Israel being accused of Arafat's death.
"Instead of launching baseless accusations against Israel, the Palestinians would be better to question those in Arafat's entourage who had an interest in his death and above all getting their hands on the money he controlled," said Gissin.
Some 60 samples were taken from Arafat's remains in November 2012 and divided between Swiss and Russian investigators and a French team carrying out a probe at his widow's request.
So far, there has been no word on the French or the Russian test results.