Passports belonging to the alleged killers of a top Hamas official in Dubai are fraudulent, the British and Irish governments have said.
Ireland said the names and passport numbers of three suspects did not match anything issued by its officials.
Britain said it believed the six British passports were also fraudulent.
Arrest warrants were issued for the suspects named by police in Dubai, where Mahmoud al-Mabhouh was murdered in a hotel room on 20 January.
Two more men, one using a French passport and one using a German passport, are also suspected of playing parts in the assassination.
The French foreign ministry said it was “not able to confirm the nationality of this person”, according to AFP news
German officials said the passport number was either incomplete or wrong, Associated Press reported.
Reports have suggested that Mr Mabhouh was in Dubai to buy weapons for Palestinian Islamist movement Hamas.
I don’t know how this happened or who chose my name or why, but hopefully we’ll find out soon
Melvyn Adam Mildiner
Hamas have accused Israeli agents of killing him.
A day after Dubai police announced the names of the Irish suspects as Gail Folliard, Evan Dennings and Kevin Daveron, a spokesman for Ireland’s Department of Foreign Affairs said: “We are unable to identify any of those three individuals as being genuine Irish citizens.
“Ireland has issued no passports in those names.”
The passport numbers had the wrong number of digits and did not contain letters as authentic passports do, he added.
“These purported passports are false. These are not genuine passports.”
Gordon Corera, BBC News, security correspondent
Taking on a different identity is a regular occurrence for intelligence agents on undercover missions. Spies often have a stack of passports under different names ready to go.
It is more difficult and potentially more sensitive to take on a different nationality – and particularly if you are found out, since it can lead to diplomatic protests.
One way of falsifying a nationality is to forge a passport or insert a new identity and picture into a stolen passport.
But a more effective way can be to steal a real person’s identity and get a real passport in their name.
There are suspicions that Israel’s Mossad was involved, particularly since it has used foreign passports for previous operations including the 1997 attempt on the life of Khaled Meshal in Jordan, when two operatives used Canadian passports.
The Britons were named as James Leonard Clarke, Stephen Daniel Hodes, Paul John Keeley, Michael Lawrence Barney, Jonathan Lewis Graham and Melvyn Adam Mildiner.
But the Foreign and Commonwealth Office said: “We believe the passports used were fraudulent and have begun our own investigation.
“We have informed the authorities in the UAE that this is the case, and continue to co-operate closely with the Emiratis on this matter.”
One man with the same name as a British suspect said it was not him.
Melvyn Adam Mildiner, a British man living in Israel, told Reuters news agency: “I am obviously angry, upset and scared – any number of things. And I’m looking into what I can do to try to sort things out and clear my name.
“I don’t know how this happened or who chose my name or why, but hopefully we’ll find out soon.”
He added that he had his passport with him.
Two Palestinian suspects were being questioned about the murder. Police said they had “fled to Jordan” after the killing, without releasing their names.
Officials in Dubai said the team appeared to be a professional hit squad, probably sponsored by a foreign power.
They released CCTV footage which they said showed some of the suspects in disguises, including wigs and false beards, in the hotel near Dubai’s international airport.
The suspects allegedly trailed Mr Mabhouh when he arrived in Dubai from Syria, paid for everything in cash and used various mobile phones.