Jeremy Corbyn’s closest aide would have to be sacked for reasons of national security should the Labour leader become Prime Minister, the former head of MI6 warned last night.

Sir Richard Dearlove said that Seumas Milne had ‘no chance’ of getting security clearance to see classified documents because of his links to terrorists and political extremist going back decades.

The warning came in response to today’s Mail on Sunday investigation uncovering startling new evidence of Mr Milne’s long-standing ties to Middle Eastern militia groups and his support for Kremlin-sponsored aggression.

Sir Richard, who led the Secret Intelligence Service MI6 from 1999 to 2004, told The Mail on Sunday that if Labour won an Election, Mr Corbyn would have to cut off all contact with his director of communications and strategy.

Jeremy Corbyn’s closest aide would have to be sacked for reasons of national security should the Labour leader become Prime Minister, the former head of MI6 warned last night (pictured: Seumas Milne with Jeremy Corbyn)

Should Mr Corbyn insist on retaining Mr Milne in power, it would plunge the Labour Party into further crisis.

David Cameron’s press chief Andy Coulson was denied security clearance, contributing to his eventual departure.

Former Guardian journalist Mr Milne, 60, has been described by Labour insiders as ‘Corbyn’s brain’ because of his enormous influence.

‘Anyone with his sort of background could not be let anywhere near classified information. It would be out of the question,’ Sir Richard said. ‘That means Corbyn could not make the judgments and decisions a PM has to make unless he stopped consulting him.’

Coming in the week that nine Labour MPs quit the party over its handling of claims of anti-Semitism, our investigation shows that Mr Milne has supported militant groups dedicated to destroying Israel by violent means since he met members of terrorist organisations on a trip to the Middle East while he was studying at Oxford University.

In his own writings, he has repeatedly expressed support for militants – first, the Palestine Liberation Organisation and, more recently, Hamas, the brutal Islamist group that controls the Gaza Strip. Yesterday, a Labour source insisted he did not endorse Hamas’s extremist charter, which says it is legitimate to use violence to wipe Israel off the map.

Pictured: Sir Richard Dearlove

Moderate Labour MPs say Mr Milne has an ‘undue influence over Mr Corbyn’, especially over the party’s handling of anti-Semitism. One prominent Jewish MP called it ‘appalling’, adding: ‘The only reason I am reluctant to put my name to this is that I know that, if I do, I will get death threats.’

Our investigation also documents how Mr Milne has parroted Kremlin propaganda since he became business manager at the hardline newspaper Straight Left in 1979. And he has defended Russian aggression against its neighbours since the 1979 invasion of Afghanistan to its continuing campaign against Ukraine.

When Mr Milne chaired a conference in the Black Sea resort of Sochi, addressed by President Putin, weeks after Russia’s illegal annexation of the Crimea in 2014, all of his expenses were paid for by the organisers, a think-tank close to Putin’s government.

Professor Anthony Glees, director of the Centre for Security and Intelligence Studies at Buckingham University, said that Mr Milne would be certain to fail the ‘developed vetting’ background checks required for anyone given access to classified documents.

This would mean that if Mr Corbyn refused to sack him after a Labour Election victory, it would cause ‘a constitutional crisis’.

Agreeing with Professor Glees, Sir Richard said: ‘An elected prime minister has a right to see everything – and he or she needs to, in order to do the job. But Mr Milne would not pass developed vetting, and so would be denied access to classified papers.’ He said the head of MI5 would have to tell Mr Corbyn to cut off contact with him – and then resign if he refused to act.

Former Guardian journalist Mr Milne, 60, has been described by Labour insiders as ‘Corbyn’s brain’ because of his enormous influence

Prof Glees said keeping Mr Milne would also mean that Britain would be deprived of vital intelligence from our allies, because they would not take the risk of sharing information if there were any risk that Mr Milne might see it.

Yesterday, a high-level former US intelligence official and respected Washington security commentator supported this view. John Sipher, the former head of the CIA’s Russia division, said he had ‘no doubt’ that keeping Mr Milne would damage Britain’s special relationship with America.

‘If there was a legitimate concern that Mr Corbyn would include Mr Milne in intelligence-related discussions, the US would likely share less of its most sensitive information,’ he said.

A Labour source said Sir Richard’s concerns were ‘smears’ designed to damage Labour’s electoral chances, and that he had ‘no credibility’ because he helped produce the intelligence that led to war in Iraq.