Boris Johnson has condemned the state-sponsored assassination of Alexander Litvinenko, branding the murder "horrific".
The London Mayor said there should be the "strongest possible diplomatic response" against those behind the killing.
His comments come after a public inquiry found that the radioactive poisoning of the former KGB officer in 2006 was "probably" sanctioned by Russian President Vladimir Putin.
Mr Johnson told the Press Association: "I think that the idea you could have state-sponsored assassinations on the streets of London is absolutely repugnant, horrific.
"You have got to have the strongest possible diplomatic response and move against those who have been responsible for that murder."