Asked whether the UK government has decided too quickly that Russia was behind the near-deadly attack, he said: "No. It is important to understand that this is an overall picture.
"It's not just the fact that we know that this neurotoxin is of a type that was originally developed in Russia, but also that our intelligence services know that the Russian government has kept a secret program of the Novichok nerve agent.
"They have even experimented on how best to administer small amounts of Novichok to kill people, and how people like Mr Skripal are being targeted by the Russian state and that there is even a history of state-sponsored killings in Russia, and therefore we have a very clear assessment that it is most likely an attack on behalf of the Russian state."
Speaking to Deutschlandfunk, he added: "We've shared our evidence with our allies, and obviously our allies have found this evidence convincing, and in front of the public we said that our intelligence services know that the Russian authorities experimented earlier on how best to use this nerve poison to kill people.
"So the evidence is clear enough. We have a strong belief that it was most likely an attack by the Russian state, and so we all had to act together."
Sir Sebastian said this is an established pattern of behaviour of the Russian government.
"We've seen hacker attacks on our ministries, we've seen data theft, interference in our policies and elections, spreading fake news to split our societies and even fuel hatred among our communities," he said.
Claiming that despite the chief of the investigating laboratory in Porton Down saying he cannot confirm if the poison was made in Russia, Sir Sebastian said "the overall picture is quite convincing".
He also said the Russian government is "clearly" pushing against the Chemical Weapons Convention by failing to disclose the programme.
As reported, Russian ex-spy Sergei Skripal, 66, and daughter Yulia Skripal, 33, remain critically ill in hospital, after they were found unconscious on a bench in the Wiltshire city on March 4. The UK government says they were poisoned with a nerve agent of a type developed by Russia called Novichok and PM Theresa May said she believed Moscow was "culpable."
UK expelled 23 Russian diplomats as part of a "full and robust" response – prompting Russia's Foreign Minister Sergei Lavrov to say it will "certainly" expel British diplomats in response.
On March 16, Nato Secretary General Jens Stoltenberg said the alliance had "no reason to doubt the findings and assessments by the British government" which suggested Russian responsibility. He said the "UK is not alone" and Nato allies gave "strong political support" to Britain, following a joint statement from the US, France and Germany backing Mrs May's government and a pledge of support from Australia.
British Foreign Minister Boris Johnson said that the U.K.'s government believes that it was likely Putin's decision to direct the use of a nerve agent on the streets of the U.K.
On March 26, U.S. President Donald Trump ordered the expulsion of 60 Russians from the United States, including 12 people identified as Russian intelligence officers who have been stationed at the United Nations in New York.