Jeremy Corbyn's most influential Commons adviser has been barred from entering Ukraine on the grounds that he is a national security threat because of his alleged links to Vladimir Putin's 'global propaganda network'.

Former communist Andrew Murray, Mr Corbyn's chief political adviser, was slapped with the three-year ban in June on the advice of the country's security service, the SBU.

But last night Mr Murray, who has played a key role in a campaign to support pro-Moscow separatists in Ukraine, angrily denied being part of any 'so-called propaganda network'.

Andrew Murray (left), Jeremy Corbyn's chief political adviser, was slapped with the three-year ban in June on the advice of the country's security service

The extraordinary development comes after it was revealed that Mr Murray has been working in the Labour leader's office for the past year without security clearance as the Commons authorities have failed to grant his request for a pass.

The Mail on Sunday has been told by a senior Parliamentary source that Mr Murray's authorisation has been held up by 'vetting problems'.

Commons security advisers take their decision based on advice from Government agencies, including the police and intelligence sources, and passes are usually issued within a few weeks.

Mr Murray, who was a member of the Communist party for 40 years, helped to launch a campaign in 2014 called Solidarity With The Antifascist Resistance In Ukraine (SARU) to protest at the West's backing of the Kiev regime. Russia made its first incursions into Ukrainian territory that year.

After President Putin annexed Crimea and backed pro-Moscow separatist forces in the self-declared Donetsk and Luhansk People's Republics, Mr Murray used SARU events to describe the conflict as 'a war waged by the Kiev government' against 'peoples exercising their right to self-rule'.

The letter banning Mr Murray from Ukraine. He has played a key role in a campaign to support pro-Moscow separatists in Ukraine

Instead of blaming Putin's regime for the bloodshed, Mr Murray has pointed the finger at the pro-Western Kiev government for obeying the 'dictates' of the EU and Nato.

He also questioned whether Moscow was responsible for the shooting down of the Malaysia Airlines flight MH17, killing all 283 passengers and 15 crew, over Eastern Ukraine in that year.

This newspaper spoke to a serving SBU officer in Kiev this weekend who claimed that Mr Murray had been put on the banned list because he was 'regarded as being part of Putin's global propaganda network, peddling Russian lies, particularly about Crimea and the war in Eastern Ukraine'.

He added: 'It is extremely unusual for us to ban anyone from entering the country, especially a Briton.

'It is not a step we take lightly and it means he is considered a potential threat to our national security.'

In a formal statement, the SBU said: 'We made a decision to ban the entrance of the citizen of Great Britain [Mr Murray] starting from June 2018 for 3 years based on the law of Ukraine about the legal state of foreign citizens ... in order to insure the national security of Ukraine'.

Mr Murray, the chief of staff to Len McCluskey at the Unite union, is on secondment to Mr Corbyn's office, where sources said he wields 'significant influence'.

Mr Murray only left the British Communist party to join Labour when Mr Corbyn took over as leader in 2016. He has defended the Russian tyrant Stalin, suggesting his regime was better than living in the West, and expressed 'solidarity' with North Korea.

Mr Corbyn, whose socialist beliefs came under close scrutiny earlier this year when he initially questioned the Government's belief that the Kremlin was responsible for poisoning former Russian double-agent Sergei Skripal and his daughter Yulia in Salisbury, relies heavily on Mr Murray for advice and has praised his 'enormous abilities'.

Mr Murray (middle) has questioned whether Moscow was responsible for the shooting down of the Malaysia Airlines flight MH17, killing all 283 passengers and 15 crew, over Eastern Ukraine

At a SARU event in November 2014, Mr Murray referred to the shooting down of flight MH17 four months earlier.

He described the crash as 'a propaganda point that will be established in people's minds and then the propaganda bandwagon moves on to try to obscure what is actually going on'.

The Russian government denies responsibility for the crash. But in October 2015, international airline investigators concluded that the airliner had been downed by a Buk surface-to-air missile, launched from pro-Russian separatist-controlled territory, which had been transported from Russia on the day of the crash.

The Stop The War campaign group, which Mr Murray founded, has also been accused of peddling Kremlin propaganda.

On July 21, 2014, the group published an online article comparing the downing of MH17 to the death of Archduke Franz Ferdinand, which led to the First World War. Leaked emails from the Kremlin show that Vladislav Surkov, an aide to the Russian president, had instructed Russia's agents of influence to make a 'comparison of the collapse of the Malaysian Boeing with the assassination of Franz Ferdinand' to muddy the waters over whether Russia was responsible. At the time, Stop The War was chaired by Mr Corbyn and Mr Murray was its deputy president.

Mr Murray is one of two Corbyn aides working in the Commons without security clearance

In an interview last year, Mr Murray also said Russia's intervention in the Middle East was 'miniscule compared with the serial and disastrous interventions of the Western powers'.

Mr Murray is one of two Corbyn aides working in the Commons without security clearance.

The Labour leader's private secretary, Iram Awan, has not been granted a pass for nine months – allegedly because of concerns about her associates.

Ms Awan was an activist with the Left Unity pressure group before she was called up to work for Mr Corbyn late last year. She has routinely been escorted through security at Parliament by members of Corbyn's team, while Mr Murray has been seen slipping into the Commons via a back entrance.

The authorities are investigating whether any rules have been broken because it is against Parliamentary rules for staff to use visitors' passes in order to work.

Commons passes may be refused if there are security concerns including evidence that 'an individual may be susceptible to pressure or improper influence'.

A spokesperson for Mr Murray said: 'Andrew completely rejects the suggestion of being part of a so-called propaganda network and is a frequent critic of Putin and his government. He has never been to Ukraine, nor has any plans to visit the country'.

Asked about the failure to receive his Commons pass, the spokesperson said: 'We don't comment on staffing matters.'

A Labour source said: 'We have not been told that any member of staff has been refused a pass or any reasons why any pass may not yet have been granted.'