Mahmoud al-Mabhouh, the Hamas official assassinated in Dubai, was betrayed by a close associate, the emirate's police chief claimed as it emerged today that the Palestinian's murderers used more fake Irish passports.

Lieutenant General Dhahi Khalfan Tamim described whoever leaked details of Mahbouh's arrival to his assassins as "the real killer", Abu Dhabi's al-Khalij newspaper reported. Tamim said last week he was 99% certain Israel's Mossad secret service was responsible.

Mabhouh, said by Israel to have been smuggling Iranian weapons and money into the Hamas-controlled Gaza Strip, was murdered in his room at the al-Bustan Rotana hotel in Dubai on January 19. Israel has neither confirmed nor denied its involvement.

Tamim, voicing suspicion of an "agent" in Hamas's ranks, urged it to investigate. But the Islamist movement has blamed its Fatah rival, which controls the West Bank, for helping the alleged Israeli hit team. Two Palestinians from Gaza who once worked for Fatah security are in custody in Dubai after being handed over by Jordan.

Nahru Massoud, a senior figure in the Izzedine al-Qassam brigades, the military wing of Hamas, has denied he was being investigated for involvement. He had been in Abu Dhabi but left the UAE before Mabhouh's murder, he told Hamas's al-Aqsa TV from Syria. Fatah officials had claimed he was under arrest in Damascus.

In Gaza, a Hamas MP, Salah Bardawil, said on Saturday that Mabhouh had unwittingly helped his killers by making travel plans online and discussing them on the phone, implying he was under surveillance by the Mossad. The claim was denied by Mabhouh's brother Fayek.

Dubai police now say up to 18 suspects used altered British, Irish, French and German passports before the killing. Officials said today that at least two more fraudulent Irish passports had been linked to the killing and that some of the suspects had visited the city on a reconnaissance mission.

Tamim described the murder as "no longer a local issue, but a security issue for European countries".

David Miliband, the foreign secretary, and his Irish counterpart, Michael Martin, are both due to meet the Israeli foreign minister, Avigdor Lieberman, tomorrow in Brussels, where the passports issue is certain to be raised. Britain has insisted it had no prior knowledge of the assassination or the fraudulent use of its passports.

The Foreign Office said it did "not recognise" a Sunday newspaper report claiming a British minister had been briefed that Israeli immigration officials had copied the passport details. "The defrauding of British passports is a very serious issue," a spokesman said. "The government will continue to take all the action that is necessary to protect British nationals from identity fraud."

UK officials have said no action is likely before the completion of an investigation into the affair by Soca, the Serious Organised Crime Agency.

The National, the UAE's leading English-language paper, yesterday described Britain's outrage over the passports as "less full-throated than one might have expected."

It added: "Israel's wilful violation of another nation's sovereignty to commit a murder deserves all the indignation it garners. But that so many passports have been used fraudulently is not just a matter of national shame, it is a matter of global security. There are many questions that remain. What is abundantly clear, however, is that the assassination plot is as much a European problem now as a Middle Eastern one."