MOSCOW - 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 to British channel ITV after he cast his vote on Sunday (March 18) in the Russian presidential election, Mr Gorbachev said "time will tell" who was behind the attack.

The 87-year-old's comments came as London and Kremlin stepped up their war of words over the March 4 attack in the quiet town of Salisbury.

He was the last leader of the Soviet Union before it collapsed in 1991, leading to the end of the Cold War.

He has said that Russian President Vladimir Putin was a "worthy president".

Relations between London and Moscow have crashed to a post-Cold War low over the Salisbury attack, the first known offensive use of a nerve agent in Europe since World War II.

Inspectors from the world's chemical weapons watchdog, the Prohibition of Chemical Weapons (OPCW), on Monday (March 19) began examining the poison used on the Skripals.

They began running independent tests on samples taken from Salisbury to verify the British analysis, said an OPCW source speaking on condition of anonymity, reported Reuters.

"The team from The Hague will meet with officials from the Defence Science and Technology Laboratory and the police to discuss the process for collecting samples, including environmental ones," Britain's Foreign Office said.

The identification of Novichok as the weapon has become the central pillar of Britain's case for Russia's culpability.

London says the Soviet-designed military grade nerve agent Novichok was used to target Skripal and on Thursday Britain, France, Germany and the US issued a joint statement blaming Russia for the first offensive use of chemical weapons in Europe since World War II.

Russian President Vladimir Putin, who easily won another six-year term on Sunday, said the claims that Moscow was behind the attack were nonsense and that Russia had destroyed all its chemical weapons.

The Kremlin said on Monday that London would either have to back up its assertions that Russia was behind the poisoning of a former spy and his daughter in Britain with evidence or apologise "sooner or later".

Kremlin spokesman Dmitry Peskov told a conference call with reporters that Britain's allegations were "difficult to explain... groundless and slanderous".

Its ambassador to the EU, Vladimir Chizhov, suggested at the weekend that Britain itself may have been the source of the chemical, reported BBC News.