"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", Aitkenhead said.
However, Aitkenhead added that Novichok required "extremely sophisticated methods to create, something only in the capabilities of a state actor".
"We hope that during those discussions a full stop will be placed on (the issue of) what happened" to Skripal, Putin said ahead of the meeting on Wednesday of the Organisation for the Prohibition of Chemical Weapons (OPCW).
He noted that Porton Down was responsible for identifying the nerve agent and providing scientific information to the government, but not for concluding where the poison was made.
Moscow has angrily denied any involvement.
Putin said the type of the nerve agent used in the March 4 poisoning could be made in some 20 nations.
"It is our assessment that Russian Federation was responsible for this brazen and reckless act and, as the worldwide community agrees, there is no other plausible explanation", the spokesperson said.
United Kingdom experts say that the Skripals were exposed to A234 nerve agent allegedly belonging to the "Novichok" group of nerve agents developed in the Soviet Union.
The tensions also are an outgrowth of USA sanctions on Russian Federation for meddling in the 2016 US elections.
Putin said he was astonished at how rapidly the anti-Russian campaign has been whipped up on the backdrop of the Skripal case.
Britain blames Russian Federation for the March 4 attack on Skripal and his daughter Yulia, a claim that Moscow fiercely denies.
"We hope to discuss the whole matter and call on Britain to provide every possible element of evidence they might have in their hands", said the Russian ambassador to Ireland, Yury Filatov.
The first use of chemical weapons in Europe since World War II has chilled Moscow's relations with the West, as both sides have expelled scores of diplomats.
Chizhov says "Russia clearly had no motive" to attack Skripal, who was convicted of spying for Britain but freed in a 2010 spy swap.
The March 4 poisoning of Sergei Skripal and his daughter in the English city of Salisbury sent shockwaves across Europe.
Many countries have expelled Russian diplomats from there countries and Russia has retaliated by expelling 60 USA diplomats and closing the US consulate in St. Petersburg, Russia.
The Trump administration's decision followed the expulsion of 23 Russian diplomats from the U.K. Russia responded to those expulsions by kicking an equal number of British officials out of Russia.