JEREMY Corbyn has again refused to blame Moscow for the Salisbury poison gas attack, saying he wanted to see "incontrovertible evidence" first.

The Labour leader called for the role of the Organisation for the Prohibition of Chemical Weapons to be strengthened when it carried out investigations into such attacks.

Relations between the UK and Russia have been plunged into the deep freeze following the poisoning of Russian former spy Sergei Skripal and his daughter Yulia with a military grade nerve agent.

Mr Corbyn, who has been given intelligence briefings on the incident, told BBC One's The Andrew Marr Show that he wanted further proof.

"I want to see incontrovertible evidence of it," he declared.

Asked if he agreed with John McDonnell, the Shadow Chancellor, 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."

Appearing on the same programme Boris Johnson said he was disappointed by the Labour leader’s lack of vigour in believing UK intelligence.

The Foreign Secretary said: “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, is quite extraordinary and a sort of blindness to reality that I find very, very perplexing."

On the state of the UK-Russia relationship, Mr Johnson said the UK wanted to engage but noted: "I'm afraid the Russians give us every possible signal and evidence that we also have to beware."

Asked if he was concerned about there being revenge cyber-attacks on the NHS and electrical facilities, he replied: “We have to take every possible precaution."