President Vladimir Putin has warned that Moscow’s enemies will “swallow poison”, as British authorities reveal the substance used in the attempted assassination of a former Russian spy.

In an angry address aimed at the West’s sanctions over Moscow’s annexation of Crimea, Mr Putin on Thursday (AEDT) said efforts to contain Russia were “illegitimate and unfair”.

“Those who serve us with poison will eventually swallow it and poison themselves,” he said in an interview with Russian state television.

The grim threat came just hours before Scotland Yard confirmed former Russian double agent Sergei Skripal and his daughter Yulia were the victims of a nerve agent.

Metropolitan Police counter-terrorism chief Mark Rowley confirmed the attack on Mr Skripal, 66, and his daughter, 33, in the English city of Salisbury was being treated as attempted murder.

Sergei Skripal attending a hearing at the Moscow District Military Court in 2006. Photo: Getty

Mr Rowley said police believed the pair, who remain in a critical condition after they were found unconscious on Sunday, were specifically targeted.

He said a police officer who attended the pair at the scene was also affected by the substance.

Mr Rowley said government scientists had identified the specific substance used, but would not make that information public at this stage.

“This is being treated as a major incident involving attempted murder, by administration of a nerve agent,” he said.

“Having established that a nerve agent is the cause of the symptoms … I can also confirm that we believe that the two people who became unwell were targeted specifically.”

Russia has previously denied any involvement in the incident, while Foreign Secretary Boris Johnson said Britain would respond “robustly” if Moscow was found to be involved.

Yulia Skripal is also in a critical condition. Photo: Facebook

Mr Putin, who presented a sweeping array of new Russian nuclear weapons last week, on Thursday voiced hopes that nuclear weapons will never be used, but warned that Russia will retaliate in kind if it comes under a nuclear attack.

“The decision to use nuclear weapons can only be made if our early warning system not only detects a missile launch but clearly forecasts its flight path and the time when warheads reach the Russian territory,” he said.

“If someone makes a decision to destroy Russia, then we have a legitimate right to respond.

“Yes, it will mean a global catastrophe for mankind, for the entire world. But as a citizen of Russia and the head of Russian state I would ask: What is such a world for, if there were no Russia?”

In the same interview, President Putin lavished praise on US President Donald Trump, describing him as a “great communicator”.

Asked if he was disappointed with Mr Trump, Mr Putin told state TV that he had no such feeling, adding he had a very positive impression of Mr Trump, whom he met on the sidelines of international summits last year.

Mr Putin said Mr Trump was easy to talk to. A man with whom “you can search for a compromise”.

-with AAP