Relations between Israel and Britain slid towards a 20-year low yesterday as London delivered a stern warning over the "outrage" of faked UK passports being used by alleged assassins in Dubai.
Ministers are assiduously refraining from making "any accusations" over the killing of Mahmoud alMabhouh, a senior figure in Hamas, the Palestinian Islamist movement, in Dubai.
However, Ron Prosor, the Israeli ambassador, was summoned to the Foreign Office yesterday to be told of Britain's determination to "get to the bottom" of an "extremely serious" incident. David Miliband, foreign secretary, said: "We wanted to give Israel every opportunity to share with us what it knows about this incident. We hope and expect that they will cooperate fully."
These exchanges could be the prelude to a more damaging turn in UK-Israeli relations. Mr Miliband is said to be deeply disturbed by allegations over the misuse of British passports. If proven true, this could damage the framework of trust underpinning the alliance with Israel.
If ministers see conclusive proof that Mossad, the Israeli overseas intelligence agency, was linked to the killing, this would also damage the relationship.
For his part, Dubai's police chief, Lt Gen Dhahi Khalfan Tamim, said he was almost completely sure of Mossad's responsibility. "It is 99 per cent, if not 100 per cent, that Mossad is standing behind the murder," he told the Abu Dhabi-based National newspaper, without elaborating on the evidence.
Mr Miliband said that "interference with British passports is an outrage". But he added: "It's very, very important that we don't make accusations until we know that they're well founded." The foreign secretary asked Mr Prosor to "take back to his government the seriousness with which we are addressing the situation".
Yesterday Sir Peter Ricketts, the Foreign Office permanent secretary, urged Mr Prosor to disclose everything that Israel knows about the episode and help an investigation led by the Serious Organised Crime Agency.
In a sign of mounting diplomatic tension, however, Mr Prosor emerged from the meeting - to which he had been "invited" at short notice - saying he had been "unable to add additional information". Israel says there is no evidence to show Mossad's involvement in the killing, but officials have offered no firm confirmations or denials, in line with their usual policy.
The alleged killers of Mabhouh included 11 people holding fake European passports. Six Britons, who all have links to Israel, allegedly had their passports cloned, with their names alongside fake photographs and signatures.
While officials say that Israel and London have an "alliance", key issues have strained their relations for many years, notably the expansion of Jewish settlements on the occupied West Bank. But Israel and Britain have close commercial ties and also co-operate closely on some high-priority foreign policy issues, particularly Iran's nuclear ambitions.
Diplomats stress that relations are not yet in a "deep freeze". Given the early stage of the investigation, an official diplomatic rebuke would be seen as premature. But it is recognised at senior levels that this may prove necessary. Officials have been studying precedents, including a espionage controversy that led to the expulsion of two Israeli diplomats from London in 1987.
The Conservatives called on Mr Miliband to win guarantees from Israel over its future conduct. David Cameron, Conservative leader, said: "At the very least, we need some assurances about the future to make sure whatever has happened in the past can't happen again."
Mr Miliband will take up the issue with his Israeli counterpart, Avigdor Lieberman, in Brussels on Monday.
Meanwhile, the Dubai police investigation is focusing on identifying five other suspects, all believed to have European passports, and questioning two Palestinians who were arrested in connection with Mabhouh's killing. Hamas officials are believed to have claimed that these suspects belong to the rival Fatah party of Mahmoud Abbas, the president of the western-backed Palestinian Authority.
Additional reporting by Tobias Buck in Jerusalem and Simeon Kerr in Dubai