Police marksmen are reported to be guarding the Queen and Tony Blair amid fears of a terrorist assassination plot.
Concerns over their safety has been increasing in recent weeks, as military tensions in the Middle East grow. The Queen and prime minister already have teams of bodyguards, but the police feel these might not be enough to prevent an assassin.
In the future the Queen's entourage will be more like that of the US president, with armed officers taking up strategic positions on rooftops surrounding any engagement. The measures come in the wake of the discovery of ricin poison in a raid in North London. The toxic gas is not suitable for mass murder, but is said to be ideal for the assassination of a single individual.
"Terrorists are looking at ways to creating maximum propaganda with the resources they have," said one expert, adding: "An attack on the royals would generate worldwide publicity."
Security for other politicians and members of the royal family is also being beefed up. The Home Office said that "a heightened level of threat from a number of sources" has led to a comprehensive review of arrangements.