Apparently the poisoning of Alexander Litvinenko in November was the most recent ( and final ) attempt, perhaps in a long string:

BBC NEWS | UK | 'Multiple attempts' on Litvinenko

Moreover, the former head of the FSB has said that Mr Litvinenko is a "traitor" who "would have been sentenced to death" in the Soviet era.

BBC NEWS | UK | Litvinenko 'a traitor' - ex-boss

Most Russians seem to feel Boris Berezovsky, an Oligarch exiled to London, was more likely behind the poisoning than Vladimir Putin. Russian news is reporting that London is on the brink of panic. Scotland Yard sees things differently:

BBC NEWS | UK | Lugovoi is 'most likely poisoner'