THE Treasury civil servant who issued an email leaking sensitive information about Royal Bank of Scotland's plans to leave the country in the event of a yes vote had links to the head of Better Together campaign, it can be revealed.

Last year the Sunday Herald told how the email - stating RBS had plans to move to London in the event of independence - had been sent to journalists by the Treasury's 'Head of Scottish Referendum Communications'. It was sent the week before the referendum, triggering headlines which were widely seen as a blow to the yes campaign.

The communication was also issued while the RBS board was meeting to discuss the matter and before the bank had first made a statement to the financial markets, breaching trading rules.

Now the civil servant who issued the communication can be identified as Robert Mackie, the son of Catherine MacLeod, who was a special adviser to Better Together leader Alistair Darling when he was Chancellor of the Exchequer. It is understood the link will be highlighted in former first minister Alex Salmond's autobiography, which is being published this Thursday.

Last night the Treasury insisted that Mackie only happened to be on duty at that time, despite his title of head of referendum communications. It also stated the email was a "reactive statement" issued on the instructions of senior HMT officials and Ministers - but refused to provide further details of who exactly approved the email.

SNP MSP Kenneth Gibson said: "These revelations leave the Treasury with serious questions to answer about its conduct during the referendum.

"The people of Scotland deserve the truth on this matter."

Gibson, the convenor of Holyrood's finance committee, is pushing for it to issue an invitation to Sir Nicholas Macpherson, the Permanent Secretary to the Treasury, to attend the committee and explain his actions during the referendum campaign.

He said he was concerned that Macpherson abandoned the notion of civil service impartiality during the run up to the referendum.

The finance committee has already called for greater transparency and accountability in discussions between the Scottish and UK Governments on further fiscal devolution.

Gibson said: "There are questions about Sir Nicholas' role as the independence vote approached. As we go forward and look at the possibility of new financial powers being rolled out in Scotland the relationship between Holyrood and the Treasury should be one of transparency and trust."

In the run-up to the referendum vote, Macpherson had been criticised by the SNP after taking the unusual step of releasing his formal advice on the SNP's preferred option of a currency union. In a memo to Chancellor George Osborne he said that currency unions were "fraught with difficulty" and added: "I would advise strongly against a currency union as currently advocated, if Scotland were to vote for independence."

Earlier this year Macpherson, the Permanent Secretary to the Treasury, reportedly said that the normal rules of civil service impartiality "do not apply" in such an "extreme case" as the referendum.

Speaking in January at the inaugural event of the Strand Group, a seminar series established by the Policy Institute at King's College London, Macpherson defended the Treasury's role in the referendum, stating: "Her Majesty's Treasury is by its nature a unionist institution. The clue lies is in the name."

Gibson added: "I want the committee to ask Sir Nicholas to come to Holyrood to explain his perspective. The Treasury cannot just change the rules governing the civil service to suit themselves."

The Treasury email on the issue of RBS plans in the event of independence was sent to journalists at 10.16pm on September 10, around 25 minutes before the RBS board meeting on the issue had finished.

It contained a response to a statement issued by Lloyds Banking Group which stated it had contingency plans to establish "new legal entities" in England in the event of a Yes vote, but also added a quote from a "Treasury source" which stated: "As you would expect, RBS have also been in touch with us and have similar plans to base themselves in London."

The following day, RBS issued a statement to the markets which confirmed its intention to "redomicile" in the event of a Yes vote, but added it would intend to retain a "significant level of its operations and employment in Scotland".

RBS chief Ross McEwan also issued a letter to staff in the morning saying the business was based in Scotland because of the "skills and knowledge of our people, and the sound business environment". He added: "I see no reason why this would change should we implement our contingency plans."

The letter also apologised to staff that the plans had first been revealed in the media saying "on this occasion, this was unavoidable."

Sir Jeremy Heywood, head of the civil service, has previously absolved civil servants of wrongdoing and rejected demands by Salmond that the matter be investigated.

A Treasury spokeswoman said: "HM Treasury issued a statement in response to a story in the Sun newspaper about Lloyds's contingency plans to move their registered HQ out of Scotland in the event of a 'Yes' vote, quoting an RBS source saying that RBS would almost certainly follow suit.

"It was clear that this story was likely to generate significant interest in - and uncertainty about - an issue with important implications for financial stability.

"In response, therefore, the Treasury press office confirmed its understanding of RBS' contingency planning. We judged that it was important to set this out - at a time when the UK financial markets were closed - given our overarching responsibility for maintaining financial stability in the UK.

"As this was outside normal office hours, this reactive statement was issued by the Treasury press officer who happened to be on duty at the time, acting on the instructions of senior HMT officials and Ministers."