A Tory former Cabinet minister has said the prospect of Jeremy Corbyn as prime minister "makes me tremble".
Sir Malcolm Rifkind, the former defence and foreign secretary, said the Labour leader's approach to national security leaves "a huge gap to be desired".
It came as Sir Malcolm said Russia was using classic attempts to obfuscate over the nerve agent attack in Salisbury.
He told BBC Radio 4's Today programme that classified intelligence shared with foreign governments had convinced them to take action against Russia.
Asked if the same information should be shared with Mr Corbyn, if it was requested by the Labour leader, Sir Malcolm said: "I don't necessarily have a strong view on that, because frankly Mr Corbyn's rather doubtful in terms of the way he deals with issues of this kind."
He added: "I am saying that Mr Corbyn's whole approach on matters of national security leaves a huge gap to be desired.
"And the idea that somebody like that might one day be in Downing Street makes me tremble, if I even thought it was serious."
Mr Corbyn had already made up his mind and was "already saying that he's on the same side, essentially, as the Russians on this particular matter", added Sir Malcolm.
He also described a UN meeting on Thursday night over the attack in Salisbury as "a classic Russian attempt to obfuscate".
He added: "They are in a very serious position, because it's not just the United Kingdom that has taken action against them.
"In an unprecedented way that didn't happen during the Cold War, 29 countries have withdrawn their diplomats.
"So what we're seeing at the moment is exactly what they did when they were found cheating with the Olympics, the Winter Olympics, and they were expelled from that.
"When they were seen to be using their troops to annex Crimea, denying it at first. It's classic Russian attempts to obfuscate and it's not working."