Boris Johnson has suggested Vladimir Putin was directly responsible for ordering a nerve agent attack on a former Russian spy on UK soil.
The Foreign Secretary said the UK Government believes it is "overwhelmingly likely" that the Russian president personally ordered the assassination attempt on Sergei Skripal, which left both him and his daughter Yulia in a critical condition in hospital.
Mr Johnson said: "Our quarrel is with Putin's Kremlin, and with his decision - and we think it overwhelmingly likely that it was his decision - to direct the use of a nerve agent on the streets of the UK, on the streets of Europe, for the first time since the Second World War.
"That is why we are at odds with Russia."
The Foreign Secretary was speaking during a visit to the Battle of Britain Bunker museum in Uxbridge with his Polish counterpart, Jacek Czaputowicz.
The UK Government has said it is "highly likely" that Russia was behind the attack but until now has not directly suggested Mr Putin was personally responsible for ordering the poisoning, which used the "military grade" nerve agent Novichok.
The Kremlin responded to Mr Johnson's claims almost immediately, saying they were "shocking and unforgivable".
Russian foreign minister Sergey Lavrov had earlier said Moscow had "stopped paying attention" to British claims about the poisoning.
He also criticised Gavin Williamson after the Defence Secretary said on Thursday that Russia should "go away and shut up".
"Maybe he lacks education," Mr Lavrov said.
UK ministers have been careful to make clear that the British response to the attack is targetted at the Russian government, not Russia as a whole.
Theresa May announced on Wednesday that 23 Russian diplomats will be expelled from the UK and a range of other measures taken against Moscow, including detaining suspected spies at the UK border, increasing checks on private flights and freezing some Russian state assets.
