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."
Britain responded to the report by announcing financial sanctions against Andrei Lugovoi and Dmitri Kovtun, who allegedly carried out the killing. Both deny involvement.
The Treasury has officially issued a "freezing order" in relation to funds owned, held or controlled by the pair under the Anti-Terroism, Crime and Security Act 2001.
It was laid before Parliament on Friday morning and came into force from midday.
The order states: "The Treasury believe that action constituting a threat to the life of one or more nationals and residents of the United Kingdom has been taken by certain persons who are residents of a country or territory outside the United Kingdom."
Britain came under pressure to confront Russia after the inquiry's findings were published on Thursday.
The barrister for Mr Litvinenko's widow warned it would be "craven" if the Government avoided substantial reprisals, while the Russian ambassador hit out at a "gross provocation".
The fall-out continued on Friday as Lugovoi rejected the inquiry as "nonsense".
Speaking to the BBC, he said: "I've seen the nonsense conclusions of your judge, who has clearly gone mad.
"I saw nothing new there. I am very sorry that, 10 years on, nothing new has been presented, only invention, supposition, rumours.
"And the fact that such words as 'possibly' and 'probably' were used in the report, means there is no proof, nothing concrete against us."
Mr Litvinenko died aged 43 in November 2006, sparking an extraordinary murder investigation.
A £2.2 million inquiry into his death was finally held last year following a long battle by his widow Marina.
In a 300-page report, the probe's chairman, Sir Robert Owen, found that Lugovoi and Kovtun were "probably" acting under the direction of Moscow's FSB intelligence service when they laced Mr Litvinenko's tea with polonium at the Millennium Hotel in London's Mayfair.
Singling out then-FSB chief Nikolai Patrushev alongside Mr Putin, the former judge wrote: "Taking full account of all the evidence and analysis available to me, I find that the FSB operation to kill Litvinenko was probably approved by Mr Patrushev and also by President Putin."
Moscow dismissed the findings, describing the inquiry as a political "whitewash".