Prime Minister Theresa May said police and prosecutors now believe the attack on Sergei Skripal was carried out by two Russian military intelligence officers who were nearly certainly acting with the approval of senior Russian officials.
The Organization for the Prohibition of Chemical Weapons confirmed Tuesday that Rowley and Sturgess were also exposed to Novichok.
The leaders added they would strengthen their activities to defend their societies against "malign state activity" and disrupt the hostile actions of foreign intelligence networks.
Investigators determined they were sickened by a Soviet-designed nerve agent called Novichok. Ms Sturgess died in hospital on 9 July.
The box, which was labelled as Nina Ricci's "Premier Jour" perfume, contained a bottle and an applicator. "But should either of these individuals ever again travel outside Russian Federation, we will take every possible step to detain them, to extradite them and to bring them to face justice here in the United Kingdom".
Prime Minister Theresa May said the two assassins were officers of the GRU intelligence service and that their actions were nearly certainly approved "at a senior level of the Russian state".
"We were right to say in March that the Russian State was responsible".
The UK on Wednesday accused two Russian men of attempting to kill a former Russian spy in England with nerve agent earlier this year.More news: Burt Reynolds dead at 82
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Metropolitan Police say they arrived at Gatwick Airport from Moscow on 2 March and stayed at the City Stay Hotel in Bow Road, east London before travelling to Salisbury on 4 March, where Mr Skripal's front door was contaminated with Novichok.
"Neither Russia's top leadership nor those in the ranks below, nor any official representatives have anything to do with the events in Salisbury", Peskov said.
Britain and dozens of other countries have kicked out scores of Russian diplomats over the incident, and Moscow has responded tit-for-tat with an identical number of expulsions.
British police released CCTV images of two Russian men they said flew in to Britain for a weekend to commit murder. She also said "novichok" which was a military grade nerve agent was planted by the two who were officers in the GRU, Russian military intelligence. This is the same Russian military intelligence agency that Sergei Skripal had worked for when he was funneling information to MI6.
I don't think that anyone can ever say that Mr Putin isn't in control of his state. And the GRU is without doubt not rogue.
A composite image showing a fake perfume box and bottle that contained the nerve agent meant to poison former spy Sergei Skripal. "It is led, linked to both the senior members of the Russian general staff and the defence minister, and through that into the Kremlin and the president's office".
Britain will brief the UN Security Council later Thursday on its latest findings, with the meeting due to open around 11:30 am (1530 GMT).
A British inquiry in 2016 concluded that Russian President Vladimir Putin probably approved Litvinenko's murder, an accusation that Moscow has always rejected.
He appealed to the public "to come forward and tell us who they are".
The Russian embassy in the United Kingdom used its Twitter account to post a series of messages aimed at undermining the credibility of the United Kingdom investigation - including comparisons to the intelligence evidence used to build the case for the 2003 invasion of Iraq.