Russia has hit back with strong rhetoric as the UK continues to blame the country for the use of a nerve agent on British soil following the alleged poisoning of a former double agent.

Sergei Skripal, a 66-year-old former Russian intelligence officer, was found in a collapsed state along with his 33-year-old daughter on a public bench outside a shopping centre in Salisbury, England, last month.

They remain in critical condition.

The UK maintains that Russia is responsible and, in a joint statement last week, the leaders of the US, Germany and France backed Britain's claim.

Russia maintains there is no proof.

"Sooner or later these unsubstantiated allegations will have to be answered for: either backed up with the appropriate evidence or apologised for," Dmitry Peskov, Kremlin spokesperson, said in a statement on Monday.

Peskov said he would not "make use of words such as escalation with the West referring to this episode".

Attempted murder

The poisoning of Skripal, who was convicted by a Russian military court of spying for the British, is being investigated by UK police as attempted murder.

Britain and its allies have called for Russia to disclose details of its Novichok supply - the poison alleged to have been used on Skripal - to the International Organisation for the Prohibition of Chemical Weapons (OPCW).

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Novichok, which was developed in the Soviet Union, disconnects the central nervous system from vital organs.

Britain announced last week the expulsion of 23 Russian diplomats following Skripal's poisoning.

Russia responded in kind.

EU foreign ministers said they took the accusations that Russia was the perpetrator of the attack "extremely seriously".

As relations continue to sour, several Investigators from the OPCW were due to visit Britain on Monday to take samples of the alleged nerve gas used.

After winning a fourth term as president in Russia's elections on Sunday, Vladimir Putin described the accusations as "complete drivel, rubbish, nonsense that somebody in Russia would allow themselves to do such a thing ahead of elections and the World Cup".

Putin maintained that Russia had destroyed all its chemical weapons and that it was ready to take part in the investigation.

Maria Zakharova, Russia's foreign ministry spokesperson, said the probable sources of the nerve agent could be Great Britain itself, as well as Slovakia, the Czech Republic, Sweden and the US.

According to Zakharova, these countries have been researching substances connected to the Novichok project since the 1990s.