The revelation means at least 12 British identities were cloned to carry out the audacious hit on Mahmoud al-Mabhouh in Dubai.
Dubai police yesterday identified 15 new suspects over the attack at a luxury hotel, bringing the total number to 26.
The assassination, which bears all the hallmarks of a spy novel, is widely believed to be the work of Israel's feared secret service Mossad.
Among the six new Britons named yesterday is Gabriella Barney, 23 - whose father Michael was among the first group of 'suspects' to be named.
The father and daughter live in Israel on dual passports.
Relatives of those named yesterday expressed their fear and worry over the development.
The other British passport owners were named as Daniel Marc Schnur, 32, Roy Allan Cannon, 62, Stephen Keith Drake, 54, Mark Daniel Sklar, 30, and Philip Carr, 35. Three Irish passport holders, three Australians and three French were also on the list.
Al-Mabhouh was electrocuted and suffocated in his hotel room on January 20.
At one point last year no fewer than nine of the suspects were in Dubai at the same time, flying in from November 6 to 8 and leaving between November 8 and 10.
Israel has repeatedly refused to confirm or deny its involvement, but Danny Danon, a leading Israeli politician, last night applauded the killing, describing as a 'very successful mission'.
Earlier this month, Dubai authorities released the names and photographs of 11 'agents with European passports' - six from the UK, three Irish, one French and one German. All four countries say the documents are fraudulent.
Within hours of the release of the second batch of names, it became clear that none of them were who they claimed to be.

He said: 'I've just come from Gaby's house and she has no idea yet. We're all a bit worried.'
Mr Sklar's mother, Rachel, speaking from her home in Staines, West London, said her son lived and worked in Israel.
'It's a major concern for us', she said, adding that neither she nor her son had been contacted by the Foreign Office.
She spoke to him yesterday after being contacted by journalists. 'He was very shocked,' she said. 'It's a bit worrying.'
Shadow Foreign Secretary William Hague called on the Government to pressure Israel for answers.
He said: 'The Government has avoided addressing the crucial issue of whether they have sought specific assurance from Israel that it will not sanction, for whatever reason, the misuse of British passports.'
This week Mr Miliband sought answers in a 45-minute private meeting with his Israeli counterpart Avigdor Lieberman.
But he told the Foreign Secretary he had 'no information to give' at this stage.
At Prime Minister's Questions Mr Durkan (Foyle) asked Mr Brown: 'Can you confirm that you condemn not only the use of false UK passports in a criminal operation but any act of state-sponsored assassination anywhere?'
Mr Brown told him: 'I can assure you that where there are questions about the misuse of British passports they have to be answered.
'We have set up and there is an investigation ongoing into the very incidents you have raised.
'I would not draw immediate conclusions without seeing the evidence and I think it is important to see the evidence on this before any further conclusions are made.
'But I do agree with you we do not support state-sponsored terrorism in any country.'
The laws on universal jurisdiction apply to all nations 'and not just one country,' Mr Brown added.
Israel has come under withering criticism from some quarters in Europe and elsewhere in the wake of the killing of al-Mabhouh, who was found dead in his Dubai hotel room on January 20.
Dubai security cameras picked up 18 members of what the country's police commander said was a hit team, adding that he was virtually certain Mossad was to blame.
The suspects were seen on CCTV disguised as tourists, following al-Mabhouh to his hotel room in Dubai on January 20.
An assassination team was then seen going to his room while a surveillance team, including a blonde woman posing as an Irish citizen named Gail Folliard, kept lookout.
After just ten minutes, the team were seen walking out of the hotel. They fled the country within two hours, having spent less than 19 hours in Dubai overall.
Al-Mabhouh was believed to have been suffocated.
Some reports have claimed that a Hamas insider was part of the hit.
Israel has not confirmed or denied that Mossad was behind the hit, hiding behind a 'policy of ambiguity'.
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