Mr Cameron told MPs that legislation backing a regulatory body could "cross the Rubicon" by writing elements of press regulation into the law for the first time.

BBC: Leveson: David Cameron and Nick Clegg at odds over plans

For eight months, Cypriot politicians failed to recognize a central truth: that while imposing losses on depositors would cross a Rubicon, turning the euro zone into a transfer union would cross an even bigger one.

WSJ: Now We Are Seeing Politics in the Raw

But he warns that this is "a Rubicon the Germans don't wish to cross" as it is "a side door into eurobonds and mutualisation of debts".

BBC: Euro 'needs federal union to survive'

Yet the recent spate of attacks will only increase the feeling in some quarters that one day Britain's bobbies may have to cross this Rubicon.

BBC: A country in the crosshairs