Boris Johnson has claimed denying state-sponsored involvement in the Salisbury attack shows a “blindness to reality” after Jeremy Corbyn called for further proof.
The Labour leader was given intelligence briefings after the attack on former Russian spy Sergei Skripal and his daughter Yulia.
Inspectors at the Organisation for the Prohibition of Chemical Weapons (OPCW) confirmed on Thursday that the toxin used in the assault was Novichok – a military grade nerve agent developed by Russia in the 1980s.
The UK’s National Security Adviser, Sir Mark Sedwill, also set out further information about why the Government believed Russia was responsible, saying only it had the “technical means, operational experience and the motive to carry out the attack”.
Mr Corbyn told BBC One’s The Andrew Marr Show he wanted further proof.
“I want to see incontrovertible evidence of it,” he said.
Asked if he agreed with his shadow chancellor John McDonnell that it was a Russian state-sponsored attack, Mr Corbyn replied: “If we are going to make a very, very clear assertion like that we have got to have the absolute evidence to do it.”
Sir Mark said Russian intelligence agencies targeted Yulia Skripal’s email account as early as 2013.
Mr Johnson said her mobile phone had been hacked.
He told Marr: “It’s quite extraordinary, in the view of the weight of evidence now – Novichok used in assassination attempts on door handles, the hacking of Yulia Skripal’s mobile phone – to continue to deny the likelihood of Russian involvement, a state-sponsored assassination attempt, I think is quite extraordinary and a sort of blindness to reality that I find very, very perplexing.”
Mr Johnson said the UK has to “take every possible precaution” to prepare for possible revenge cyber attacks on targets like the NHS and electrical facilities.