The order to carry out the Salisbury nerve agent attack came from the “highest level” of the Russian state, Britain’s Home Secretary says.

Sajid Javid stopped short of naming Russian President Vladimir Putin as the man who authorised the Novichok poisoning by the military intelligence service GRU, but said “we all know what’s at the top of the Russian government”.

Although the suspects had been identified as GRU agents Alexander Petrov and Ruslan Boshirov and arrest warrants had been issued, “the reality is we will probably never see them in the UK” because they were unlikely to leave Russia again, Javid acknowledged.

Alexander Petrov and Ruslan Boshirov are wanted for conspiracy to murder Sergei Skripal and the attempted murder of Yulia Skripal. Photo: Scotland Yard/Getty Images

UK Prime Minister Theresa May has previously said the attack was “almost certainly” approved at a “senior level” of the Russian state, while Security Minister Ben Wallace said Putin bore ultimate responsibility as head of the government in Moscow.

Javid told the BBC the GRU operated on a “very short leash from the Kremlin” and was “getting its instructions directly from the highest levels of the Russian government”.

He said of the “sickening and despicable” attack: “Unequivocally, crystal clear this was the act of the Russian state, two Russian nationals sent to Britain with the sole purpose of carrying out a reckless assassination attempt.”

Britain’s Home Secretary Sajid Javid says Britain has “considerable powers” to respond to Russia. Photo: Getty

The home secretary said the UK had “considerable powers” to respond to Russia.

“We will bring all those powers, both overt and covert, to bear on Russia and what it represents today.”

Former Russian spy Sergei Skripal, who was poisoned by a nerve agent in Salisbury, was discharged from hospital in May – more than two months after the attack.

Mr Skripal and his daughter, Yulia, were found unconscious on a bench in the southern English city of Salisbury after exposure to a military grade nerve agent, Novichok, developed by Russia.

Moscow denied any involved in the poisoning attack released in an area where hundreds of people could have been exposed.

Russian President Vladimir Putin said he was pleased Mr Skripal had been released from the hospital, more than 10 weeks after he and his daughter had been poisoned.

-AAP