SCOTTISH Tory leader Ruth Davidson is at the centre of a potentially damaging Holyrood leak inquiry.

An investigation is under way into how Davidson obtained unpublished committee evidence last week before it went to the committee's MSPs.

At First Minister's Questions on Thursday, Davidson used the document to taunt Alex Salmond over the SNP's plans on how an independent Scotland could join the EU.

She quoted from a submission to the European and External Relations Committee by Jean-Claude Piris, former legal counsel of the European Council, which said the SNP plan was unlawful.

Conservative press officers later distributed the Piris evidence to the media. However, the paper, which had been emailed to the cross-party committee's mailbox, had not yet been sent to the committee's convener or its six other members, who only received it from parliament officials 24 hours later.

A Tory source claimed: "We just got it handed to us anonymously, kind of thing. We have not spoken to the guy [Piris] himself."

The committee's SNP convener, Christina McKelvie, said she was "shocked" by Davidson's action and accused her of "disrespect".

Confirming she had begun her own inquiry, she said: "I'm not happy. I'm not happy evidence I hadn't even read was being cited.

"It was a real disrespect to the committee to use it that way."

Davidson boasted about the status of the document on Thursday. She told MSPs: "I have a copy of a new submission to the European and External Relations Committee by Jean-Claude Piris, who is the former director general of the legal service of the EU Council.

"I am happy to put the entire submission into the public domain today so that everyone can see, in black and white, a leading European expert saying that the SNP's plan is not lawful."

The paper said the SNP proposal to join the EU within 18 months of a Yes vote by an untried route, known as Article 48, was unlawful.

Instead, Piris said Scotland would have to apply under the Article 49 procedure used by all the other countries which have joined the EU, which could mean long delays.

The row is a headache for Davidson's new spin doctor, Eddie Barnes. The former journalist joined the Scottish Conservatives last week as their director of communications and strategy.

After Davidson quoted the Piris paper in the chamber, he briefed the media on its content.

A Scottish Conservative spokesman refused to reveal the source of the submission.

"This is a typical response from the SNP, trying to mask what the real issue is with technicalities which are irrelevant in comparison.

"The point remains that the Scottish Government is misleading the public on automatic EU entry, and it is the duty of opposition parties to expose this."

Piris did not respond to calls or email.