Vladimir Putin is set to extend his power in Russian Federation for another six years after winning Sunday's presidential election with a projected 76.6% of the vote, a state-run exit poll shows.
"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 United Kingdom, on the streets of Europe for the first time since the Second World War", Johnson said.
Putin is running against seven other candidates, but his main challenger Alexei Navalny has been barred for legal reasons and the outcome is in little doubt.
The run-up to Russia's last presidential election in 2012 was marked by protests across the country against Putin's return as head of state after four years as prime minister.
According to Russia's Central Election Commission (CEC), Putin obtained more than 75.01% of the votes, after counting 50% of the votes cast. But Navalny charged that the poll has been staged and voter numbers rigged.
Russian President Vladimir Putin on Sunday proclaimed his victory in the recently conducted presidential election.
Just weeks before the election, he announced that Russian Federation has developed advanced nuclear weapons capable of evading missile defences.
Casting his ballot in Moscow, Putin said he would be pleased with any result giving him the right to continue as president.
The Kremlin's longest-serving leader since Soviet dictator Joseph Stalin had nearly 77 percent of the vote with about 95 percent of the ballots counted, putting him on track for a new six-year term.
Most people who spoke to AFP said they voted for Putin, praising him for restoring stability and national pride after the humiliating collapse of the USSR. Do you think that I will stay here until I'm 100 years old? "I fear that independent observers will become objects of such an attack".
Chinese President Xi Jinping on Monday greeted Mr Vladimir Putin for being re-elected as Russia's president for a second consecutive six-year term.
The exit poll by state-owned pollster VTsIOM at 1,200 voting stations around Russian Federation projected that Putin had won 73.9 percent of the vote, up from 64 percent six years ago.
But Navalny, who risks 30 days in jail for organising illegal protests, urged a boycott.
Without constitutional reform, Putin will not be able to run for the fifth time - Russian law forbids serving more than two consecutive terms.
The vote was tainted by allegations of forced voting and election violations as footage released by a government opposition group appeared to show ballot boxes being stuffed.
"I don't think he will refuse power in 2024 even if he has had enough, he is (already) visibly exhausted", said political analyst Dmitry Oreshkin.
"It's clear to everyone who will be elected".
Mr Johnson's decision to place blame for the attack in Salisbury on Mr Putin personally came as Britain awaited Moscow's response to the expulsion of 23 of its diplomats.
Russian authorities had appealed to patriotic feelings by holding Sunday's election on the anniversary of Russia's 2014 annexation of Ukraine's Crimean Peninsula. Putin has promised to use his new term to beef up Russia's defences against the West and to raise living standards.
Ella Pamfilova, chairwoman of the Central Election Commission who was appointed to clean up Russia's electoral system, vowed to respond to complaints about being coerced to vote.
At home, Putin would face a challenge of how to diversify an economy still dependent on oil and gas and improve medical care and social services in regions far removed from the cosmopolitan glitter of Moscow.
His friend, 21-year-old modeling agent Max V, also chose not to vote, saying there weren't enough candidates to choose from.
"How would we live without him? Putin needs to finish what he started", he said.
Exit polls are not final, and official results are gradually being released.
A combination picture shows candidates in the Russian 2018 presidential election, including (top, L-R) Vladimir Putin, Pavel Grudinin, Vladimir Zhirinovsky, Ksenia Sobchak, (bottom, L-R) Grigory Yavlinsky, Sergei Baburin, Boris Titov, and Maxim Suraikin.
Election authorities said turnout nationwide Sunday was about 52 percent at 5pm Moscow time.
Russian election officials moved quickly to respond to some of the violations.