Russia has been developing the nerve agent used to poison a former spy and his daughter in Salisbury for 10 years, Boris Johnson said.

The Foreign Secretary told BBC One’s Andrew Marr Show that the UK has evidence of the Kremlin’s decade-long production and stockpiling of Novichok.

It is believed the agent was used to poison 66-year-old Sergei Skripal and his daughter Yulia in Salisbury on March 4.

The pair remain in a critical condition in hospital.

Russian spy poisoning: Military forces work on a van in Winterslow (AP)

Mr Johnson told Marr: “We actually have evidence within the last ten years that Russia has not only been investigating the delivery of nerve agents for the purposes of assassination but has also been creating and stockpiling Novichok.”

He repeated his condemnation of Russia over the attack and said: “We gave the Russians an opportunity to come up with an alternative hypothesis and they haven’t. Their response has been a sort of smug sarcasm and denial.”

He went on to say that international experts would test evidence from the scene of the poisoning in a bid to ascertain its source.

But, also speaking to Marr, Russia’s ambassador to the EU, Vladimir Chizhov, suggested the nerve agent may have come from the Porton Down laboratory, which is about eight miles from Salisbury.

Russia’s ambassador to the EU, Vladimir Chizhov, suggests nerve agent may have come from the Porton Down laboratory

He said: “When you have a nerve agent or whatever, you check it against certain samples that you retain in your laboratories.

“Porton Down, as we now all know, is the largest military facility in the United Kingdom that has been dealing with chemical weapons research.

“It’s actually only eight miles from Salisbury.”

Asked whether Porton Down was “responsible” for the attack, Mr Chizhov said: “I don’t know. I don’t have any evidence of anything having been used.”

Salisbury MP John Glen, responded to the suggestion by tweeting: “Russia’s EU Ambassador’s outrageous attempt to smear Porton Down scientists – many of whom are Salisbury residents – is so ludicrous it would be laughable if it wasn’t for the dire consequences of his regime’s nerve agents.”

The comments came as the Russian Embassy in the UK joked that Agatha Christie’s fictional Inspector Poirot should be called in to investigate the incident in Salisbury.

It said in a tweet alongside a picture of the character: “In absence of evidence, we definitely need Poirot in Salisbury!”

Mr Skripal and his daughter remain in a critical condition following the attack.

Theresa May said on Saturday that the UK would work with allies to “consider the next steps against Russia”.