The poisoning of ex-Russian spy Sergei Skripal and his daughter, Yulia, is "horrible" and "disgusting", former Soviet Union leader Mikhail Gorbachev has said.
Speaking after he cast his vote in the Russian presidential election, Mr Gorbachev said "time will tell" who was behind the attack.
The 87-year-old's comments came as Boris Johnson claimed that the UK has evidence Russia had been developing and stockpiling the nerve agent Novichok over the past decade.
The Foreign Secretary added that independent chemical weapons inspectors will arrive in the UK on Monday to test the substance used in the Salisbury attack, though the results will take at least two weeks.
Current President Vladimir Putin looks set to win a fourth term in power, with opposition leader Alexei Navalny barred from running after announcing his candidacy in December.
Mr Putin faces several other challengers, including TV personality Ksenia Sobchak, but none pose a serious threat to his power.
As Ms Sobchak cast her ballot on Sunday, she urged Vladimir Putin's critics to vote instead of boycotting.
The 36-year-old former TV star, argued against the boycott called for by opposition leader Alexei Navalny, who is barred from running.
She claimed "every extra percentage point" for Mr Putin is a result of those who refuse or do not bother to vote, and called for critics of the incumbent President to "come together".
Some critics have argued that Ms Sobchak has the tacit support of the Kremlin so that the election looks more democratic, something she denies.
Ms Sobchak is the only candidate who has openly criticised Putin in the campaign.
Authorities have spent unprecedented funds on Mr Putin's campaign ahead of the election, to ensure the 65-year-old has a strong mandate for his next six years in office.
Election monitors have already reported irregularities at voting stations across Russia, just hours after polling stations opened.
Sunday marks the first round of the election, with a second round scheduled for Sunday, April 8, should it be required.