THE information watchdog has been asked to investigate whether John Swinney improperly blocked the release of material about Prince Charles lobbying SNP ministers.

The Liberal Democrats wrote to the Scottish Information Commissioner after it emerged Mr Swinney personally intervened in a sensitive Freedom of Information (FoI) case last year.

Emails show the Education Secretary and Deputy First Minister thought it would be “better” if material about the Prince stayed secret, despite FoI cases being a matter of strict legality.

Government special advisers then appeared to cast around for a technical exemption under which they could withhold four documents Mr Swinney would “prefer were not released”.

The Commissioner, Daren Fitzhenry, is already probing the Scottish Government’s FoI regime after complaints from the media about underhand and unfair practices.

The documents involved in the latest row show Prince Charles and his office privately lobbied SNP ministers in 2012 and 2013 to loosen up the rules on teacher training to give Teach First, a charity of which the Prince is patron, a chance to win contracts.

When journalist James McEnaney asked for the correspondence under FoI, the Scottish Government initially refused, only releasing it after the matter was taken to appeal.

Now related emails released under FoI show Mr Swinney played a key role in the affair.

On June 30 last year, Mr Swinney’s private secretary depute told civil servants the Deputy First Minister thought “it would be better to see if we could not release the material relating to Prince Charles or his PS [private secretary]. He specifically referenced documents 20, 24, 25, and 26 as ones he’d prefer were not released.”

An official then emailed special adviser Kate Higgins that this might be possible by citing an exemption under the Freedom of Information (Scotland) Act 2002.

The official said: “Think we can withhold under Section 41(a) - communications with her Majesty, the Royal Family or the Royal Household”.

Ms Higgins replied: “Am sure that’s what we used last time.”

LibDem MSP Tavish Scott said there should be an urgent review into political “meddling”.

He said: “I am writing to the Information Commissioner to ask it to urgently investigate this serious matter. These new documents suggest that special advisors were interfering in the content of replies to FoI requests.

"If information scheduled for release was withheld solely on the basis that a minister would ‘prefer’ it was not, it would seriously damage public confidence in their right to information.

"There can be no meddling of this nature. The right to information is clearly defined in law.”

Green MSP Ross Greer said: “The fact Prince Charles lobbied ministers on behalf of Teach First model is bad enough, but to see the Deputy First Minister express a preference for what should be included in FoI responses takes this issue to a new and much more serious level.

“Ministers must make a statement in parliament explaining why they think bending the rules on such an important issue is appropriate. The Scottish Government has a terrible reputation on transparency and FoI and it cannot hope to sweep this latest example under the carpet.”

Tory MSP Edward Mountain added: “The evidence suggests that special advisers are routinely involved in the freedom of information process for political purposes and John Swinney himself is suppressing documents when it suits him.”

A Scottish Government spokesperson said: “The legal duty to comply with FoI legislation lies with Scottish Ministers. As Ministers are accountable it is entirely appropriate for them to decide whether they are content with proposed information releases and exemptions applied in line with the FoI Act.”