The British government has called in the Israeli ambassador to discuss the use of fake UK passports by the alleged killers of a Hamas commander in Dubai.
Gordon Brown has also ordered an inquiry into the passports, which bear the names of six British-Israelis who are not the men pictured.
Dubai police believe 11 "agents with European passports" killed Palestinian militant Mahmoud al-Mabhouh.
Israel said there was no evidence to link its secret service.
Israel's foreign minister, Avigdor Lieberman, refused to issue any formal denial in line with a "policy of ambiguity" on security matters.
He told Israeli Army Radio: "There is no reason to think that it was the Israeli Mossad and not some other intelligence service or country up to some mischief."
It is expected that the Israeli Ambassador, Ron Prosor, will meet with Sir Peter Ricketts, head of the diplomatic service, on Thursday.
Sir Menzies Campbell, former Liberal Democrat leader and member of the Commons foreign affairs committee, said the ambassador had to be questioned.
"The one institution that does know whether Mossad was involved in this matter is the Israeli government and I expect that the senior civil servant in the Foreign Office will say 'well, now's your chance to tell us one way or another'," he told BBC's Newsnight.
The Serious Organised Crime Agency has been asked to look into the fraudulent use of the passports.
It has confirmed that photographs and signatures on the passports used in Dubai do not match those on passports issued by the UK.
Prime Minister Gordon Brown said: "We have got to carry out a full investigation into this. The British passport is an important document that has got to be held with care."
The Foreign Office said the British embassy in Tel Aviv was ready to support those affected by the Hamas shooting case.
The men whose names appeared on the passports have dual British and Israeli citizenship.
They are Melvyn Adam Mildiner, Paul John Keeley, James Leonard Clarke, Stephen Daniel Hodes, Michael Lawrence Barney and Jonathan Lewis Graham. They all deny involvement in the killing.
Several of them have spoken of their shock at being implicated in the crime.
Salford-born Mr Hodes, 37, said he had not left Israel for two years and was "in shock".
"I don't know who's behind this. I am just scared, these are major forces," he told Israeli television.
The details of the suspects and their passport photos were released by officials in Dubai earlier this week.
Three of the other suspects used Irish passports.
Authorities in the Irish Republic have confirmed that while the numbers were legitimate, they did not match records for the names which had been used - Gail Folliard, Evan Dennings and Kevin Daveron.
Its Department of Foreign Affairs said officials were urgently trying to contact the three citizens who hold or have held passports with these numbers.
France and Germany have also reportedly raised doubts over the identities of two suspects who used a French and a German passport.
Mr Mabhouh was murdered in his hotel room in Dubai on 20 January.
Reports have suggested he was there to buy weapons for the Palestinian Islamist movement, Hamas.
Two Palestinian suspects were being questioned about the murder. Police said they had "fled to Jordan" after the killing and have not released their names.
Officials in Dubai, who have issued arrest warrants, 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.