Russian President Vladimir Putin has said he hoped a line could be drawn under the poisoning of former Russian double agent Sergei Skripal in Britain at a meeting of the global chemical weapons watchdog tomorrow.

Speaking at a news conference in Turkey, Mr Putin also said he was surprised by the pace of what he called an anti-Russian campaign that unleashed Britain's accusation that Russia was behind the nerve toxin attack on Mr Skripal and his daughter on 4 March.

He noted that the head of Britain's Porton Down military research laboratory had said earlier today that it was not possible to say yet where the military-grade nerve agent that struck the pair down had been produced.

Mr Putin said there were about 20 countries where such substances were made.

Mr Putin's spokesman, Dmitry Peskov, told reporters that Britain's theory on who poisoned Mr Skripal "will not be confirmed anyway."

Russia has denied being behind the attack in the southern English city of Salisbury.

Gary Aitkenhead, chief executive of the Defence Science and Technology Laboratory at Porton Down in England, said earlier that while the substance used was identified as Novichok, a source was not established.

Speaking to Sky News, he added: "We have not identified the precise source, but we have provided the scientific info to government who have then used a number of other sources to piece together the conclusions you have come to."

However, he confirmed the substance required "extremely sophisticated methods to create, something only in the capabilities of a state actor".

He added: "We are continuing to work to help to provide additional information that might help us get closer to [the source] but we haven't yet been able to do that."

After the first known use of a military-grade nerve agent on European soil since World War II, Britain blamed Mr Putin for the attempted murder, while the West has expelled around 130 Russian diplomats.

Mr Aitkenhead said the British government had "other inputs" it could use to determine the origin of the nerve agent, some of them intelligence-based.

He reiterated that the substance could not have come from Porton Down.

Russia's EU ambassador Vladimir Chizhov noted in an interview with the BBC last month that the British research lab is only 11km from Salisbury, insinuating that may have been the source.

A UK government spokesperson said: "We have been clear from the very beginning that our world leading experts at Porton Down identified the substance used in Salisbury as a Novichok, a military grade nerve agent.

"This is only one part of the intelligence picture," he said.

He added: "As the Prime Minister has set out ... this includes our knowledge that within the last decade, Russia has investigated ways of delivering nerve agents probably for assassination and as part of this programme has produced and stockpiled small quantities of Novichoks; Russia’s record of conducting state-sponsored assassinations, and our assessment that Russia views former intelligence officers as targets."

Mr Skripal's daughter Yulia is getting better after spending three weeks in critical condition due to the nerve toxin attack at her father's home in Salisbury, the hospital where she is being treated said last Thursday.

Her father remains in a critical but stable condition.

UK urged to answer Russian questions on Skripal case

Earlier, he Russian Ambassador to Ireland said if Britain chooses to ignore its questions over the Salisbury attack there is ample ground to assume it is dealing with a "grand-scale provocation, organised by London, to discredit Russia".

So far, 29 countries, including Ireland, have expelled Russian diplomats over the poisoning.

Speaking at a press conference, Yury Filatov said he wanted to make it clear that Russia has three sets of questions that it wants answered by Britain, France and the Organisation for the Prohibition of Chemical Weapons (OPCW) over the nerve agent attack.

He said these questions are not rhetorical and must be answered.

Mr Filatov said: "The only thing that's clear is that the British government put the blame on Russia without presenting any evidence to that effect".

He said his country is calling for an extraordinary meeting of the OPCW to be held tomorrow for Britain to outline the evidence it has.

Mr Filatov said Russia had no involvement in the attack and "if not Russia, somebody did, and we must look at who would benefit either criminally or politically".

He said there is a belief that some sectors of the British government would benefit.

Britain has granted the OPCW access to samples of the substance used to poison the Skripals.

UK experts have identified it as Novichok, a nerve agent believed to have been developed by Russia.

The OPCW will be meeting at its headquarters in The Hague tomorrow morning to discuss the allegations against Russia, according to the chemical watchdog. 

"The chairperson of the Executive Council... has received a request by the Permanent Representative of the Russian Federation... to convene a meeting of the Executive Council," the OPCW said. 

Additional reporting: Dyane Connor