French prosecutors have opened a murder inquiry into the death of Palestinian leader Yasser Arafat in 2004.
His family launched a case last month over claims that he was poisoned with polonium-210, a radioactive element.
Swiss scientists hired by a documentary crew say they found traces of polonium on some of Arafat's belongings.
The medical records of Arafat, who died at a military hospital near Paris in 2004, said he had a stroke resulting from a blood disorder.
However, many Palestinians continue to believe he was poisoned by Israel because he was an obstacle to peace. Israel has denied any involvement.
Others allege that he had Aids.
'Significant' polonium traces
Arafat's family lodged papers with the French authorities asking for an investigation in July.
Arafat: The Swiss claims
"Unexplained, elevated" level of polonium-210 on Arafat's clothing, keffiyeh and toothbrush
Highest levels found on items with bodily fluids
Toothbrush measured 54 millibecquerels (mBq); underwear 180mBq compared with 6.7mBq from another man's specimen underwear
More than 60% of polonium was not from natural sources
Source: Al-Jazeera TV
French officials on Tuesday said prosecutors had agreed to begin a murder inquiry, but they have yet to appoint an investigating judge.
The BBC's Hugh Schofield in Paris says the French legal system is obliged to take the matter very seriously because of its diplomatic aspect, but the medical profession is generally sceptical about claims of radioactive poisoning.
Senior Palestinian official Saeb Erekat told the AFP news agency that the Palestinian Authority welcomed the inquiry.
He said Palestinian President Mahmoud Abbas had officially requested the help of French President Francois Hollande in the investigation.
"We hope there will be a serious investigation to reveal the whole truth, in addition to an international investigation to identify all the parties involved in Arafat's martyrdom," he said.
The inquiry stems from an Al-Jazeera TV documentary broadcast early in July.
The channel commissioned Lausanne University's Institute of Radiation Physics to analyse Arafat's belongings.
Arafat's wife, Suha (photo), supplied clothing for the examination.
The scientists told the channel that they had found "significant" traces of polonium-210 present in items including Arafat's trademark keffiyeh.
Following the documentary, Suha Arafat and daughter Zawra lodged a complaint with French judicial authorities.
Their lawyers have said they want a French investigation to work alongside international inquiries being conducted by the Lausanne scientists.
Last week, the Swiss institute said it had received permission from Suha Arafat and the Palestinian authorities to travel to Ramallah to analyse his remains for traces of polonium.
Arafat led the Palestine Liberation Organisation for 35 years and became the first president of the Palestinian Authority in 1996.
He fell violently ill in October 2004 and died two weeks later, at the age of 75, in a French military hospital.
French doctors bound by privacy rules did not release information about Arafat's condition.
In 2005, the New York Times obtained a copy of Arafat's medical records, which it said showed he died of a massive haemorrhagic stroke that resulted from a bleeding disorder caused by an unknown infection.
Experts who reviewed the records told the paper that it was highly unlikely that he had died of Aids or had been poisoned.
Highly radioactive and toxic element
Present in foods in low doses
Small amounts created naturally in the body
Can be manufactured using the bombardment of neutrons
Has industrial uses such as in anti-static devices
Very dangerous if significant dose ingested
External exposure not a risk, only if ingested
Present in tobacco
Source: BBC News