An investigation into former Scottish secretary Alistair Carmichael has been dropped by the House of Commons' standards officer.

The Parliamentary Commissioner for Standards was looking into Carmichael's involvement in a leak aimed at damaging SNP leader Nicola Sturgeon.

The Orkney and Shetland MP leaked a memo between a Scottish Office official and the French consul general that alleged Ms Sturgeon had told the French ambassador she would prefer David Cameron's Conservatives to win the then upcoming 2015 UK general election.

The commissioner said: "I have established that the conduct which led to my inquiry falls outside my remit. I do not, therefore, make any criticism, or indeed any other comment, on Mr Carmichael's conduct in this affair."

She said the matter was outside her remit because Carmichael had been made aware of the memo through official Scottish Office channels, and not due to his role as an MP.

The memo was brought to the Liberal Democrat minister's attention by his special adviser during a flight between Copenhagen and the Faroe Islands on March 9 2015. It was then given to a journalist from the Daily Telegraph on April 1, with the minister's approval.

When the story became a major incident during the general election campaign, Carmichael initially denied he had any knowledge of the affair until a journalist contacted him about the memo. He later admitted he had been involved in the leak, and he was forced to forfeit his severance pay as a minister following a top-level inquiry into the matter.

The Commissioner said she did not have sufficient information from the Cabinet Office to be able to answer questions about Carmichael's role in the matter at the beginning of her inquiry.

She added: "On the same day that I initiated my inquiry, I wrote to the cabinet secretary to ask if he might release to me a full copy of the report of the leak inquiry carried out by Cabinet Office officials.

"On June 16 2015, the cabinet secretary told me that he did not think it would be appropriate to release a copy of the investigative report."

The Commissioner also used her report into the matter to suggest that a review of the code of conduct for MPs looks at whether members seeking re-election should continue not to be covered by its rules during the period of a general election campaign.

Sturgeon, who, along with the ambassador, denied the allegation in the memo, called for Carmichael to resign as an MP after he admitted being behind the leak.

The Lib Dem was later taken to court by some of his constituents who claimed he had misled voters in the run-up to the election. He was cleared after a series of hearings at a specially-convened election court in Edinburgh.

Judges ruled that it had not been proved beyond reasonable doubt that he had committed an "illegal practice" and said he was fairly elected.

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