Britain must prove Russia's involvement in spy poisoning or apologise: Kremlin

Britain must prove Russia's involvement in the poisoning of former double agent Sergei Skripal in the UK or apologise, the Kremlin said Monday.

"Sooner or later these unsubstantiated allegations will have to be answered for: either backed up with the appropriate evidence or apologised for," presidential spokesman Dmitry Peskov said.

Peskov was responding to a question about whether the exacerbation of tensions with the West had boosted Putin's performance in Russia's presidential elections Sunday.

"I wouldn't use the phrase 'exacerbation of tensions with the West'. It's a question of this stream of slander, that is hard to explain and difficult to understand the motivation for, from the British side towards Russia," he said.

Putin on Sunday rejected as "nonsense" allegations by London and its allies that Russia was behind the March 4 attack on Sergei Skripal and his daughter Yulia in the English city of Salisbury.

In response, British Foreign Secretary Boris Johnson said Moscow's denials were becoming "increasingly absurd".

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