The heir to the throne, Prince Charles, privately lobbied Alex Salmond when he was First Minister to give money and political backing to an organic food group, his own north highland food brand, a stately home, a decaying castle and a series of other old buildings in Scotland.

The Sunday Herald has obtained details of seven personal letters from the Prince between July 2007 and July 2010, and Salmond’s replies. Known informally to officials as “black spider memos” because of Charles’s handwriting style, they reveal his secret attempts to influence spending by the Scottish Government.

The release of portions of the letters under freedom of information law was strongly opposed by the Prince - but backed by Salmond. The revelations have prompted sharp criticisms of the relationship between ministers and the royal family, raising questions about the role of the monarchy in a modern democracy.

Anti-monarchy campaigners accused Charles of “political meddling” and demanded “full disclosure” of all his letters to ministers. The leading left-wing Labour MSP, Neil Findlay, labelled Salmond as “a bit of a sook” to royalty.

Prince Charles, however, insisted he was trying to find practical ways of addressing issues of public concern. Salmond dismissed the criticisms, saying “sook-ability is never an accusation that is going to stick to me”.

In May and June this year, the UK government released 44 of the Prince’s private letters to UK ministers after losing a ten-year freedom of information legal battle. The case caused the UK government to give senior royals an absolute exemption to freedom of information requests.

The Scottish Parliament, however, rejected the inclusion of a similar exemption in the freedom of information law that covers Scotland. This means that letters from the Prince to the Scottish Government can be released if it’s judged to be in the public interest.

After initially being refused, the Sunday Herald was last week given extracts from six letters from the Prince, and six replies from Salmond. The text of the rest of the letters has been kept secret.

In addition, two complete letters - one from the Prince’s private secretary and one from Salmond - have been disclosed. But an unspecified amount of other correspondence has been kept under wraps.

In one of the extracts from a letter dated June 15 2009, Charles urged Salmond to give financial aid to the Soil Association in Scotland, of which he is the patron, for an initiative promoting healthier and more sustainable food. “I wondered if it might be possible to investigate whether there could be some form of support to assist the Soil Association in avoiding having to cut back their capacity for this initiative,” the Prince wrote.

In reply Salmond pointed out that the Scottish Government was already giving the association £148,563, but promised to see if more could be done. “I have asked my officials to meet with Soil Association Scotland and discuss scope for further support in respect of opportunities identified in Your Royal Highness’s letter”, he wrote on June 30 2009.

“I have the honour to be, Sir, Your Royal Highness’s most humble and obedient servant,” Salmond continued. The former First Minister told the Sunday Herald that that was the last time he used this traditional form of words because he thought it was “inappropriate to a democratic age”.

In another letter on June 7 2008, the Prince asked Salmond to appoint experts to help his north highland food brand, Mey Selections, develop low carbon systems. On July 1 2007 Charles expressed his “heartfelt thanks” for the Scottish Government’s donation of £5 million towards the purchase of the stately home, Dumfries House, near Cumnock in East Ayrshire.

In 2008 Charles lobbied unsuccessfully for the government’s conservation agency, Scottish Natural Heritage, to be allowed to give £2 million towards renovating the derelict Kinloch Castle on the Isle of Rum. He also bid for backing to save a deteriorating historic hospital, Glen O’Dee Sanatorium in Banchory, Aberdeenshire.

Other letters requested support for Paton’s Mill in Johnstone, which was subsequently devastated by fires, and over a thousand “redundant buildings” in Caithness. The Prince also suggested a “socio-economic fund” for communities in Caithness, Sutherland and Easter Ross, linked to tidal and wave power developments in the Pentland Firth.

Republic, a group campaigning for an end to the monarchy, called for all the Prince’s letters to ministers to be published. “The Scottish Government is not a plaything for bored royals on a mission,” said the group’s chief executive, Graham Smith.

“We need full disclosure of Prince Charles’s letters so we can all see the full extent of his political meddling. Scottish voters need to know whether government policy and spending of public money are being based on the best judgement of ministers or on pressure from royals.”

Smith branded the “royal leverage” used by the Soil Association as “unacceptable”. He added: “Charles, Salmond and the Soil Association have questions to answer about their cosy relationship.”

Hugh Raven, the director of Soil Association Scotland at the time, was reluctant to comment on private correspondence. But he said he was “extremely grateful to HRH the Prince of Wales for highlighting the good work of Soil Association Scotland to the First Minister”.

Neil Findlay MSP, Scottish campaign chief for Jeremy Corbyn’s bid to lead the UK Labour party, accused the “supposed radicals” in the Scottish Government of being firm monarchists. “This reveals the former First Minister’s credentials as a rather dutiful subject who is partial to being a bit of a sook with the Royal Family,” he said.

“What is more worrying is how the First Minister appears to have gone out his way to use public money and public resources to intervene in businesses that appear to be pet projects of the Prince of Wales. If only the rest of us enjoyed such access to the government.”

But Salmond brushed aside the criticisms, suggesting that Findlay was “a less than heavyweight politician” who should not cast stones when his Labour colleagues in London had been “sooks” to the Prince.

Salmond defended the Prince, known as the Duke of Rothesay in Scotland, arguing that he had not interfered in party politics, but raised matters of public interest. “We should be pleased that Prince Charles has these concerns,” he said, suggesting it would be “ridiculous” if he were prevented from writing to ministers like everyone else.

“Ministers should respond to requests from Prince Charles or anyone else on the basis of their judgement of the public interest - this is usually done on civil service advice,” Salmond said. He had sided with Charles against official advice on Dumfries House resulting in “probably one of the best spent £5 million in Scottish history”, he argued.

Salmond added: “I am proud of supporting both of these projects just as Scotland should be proud of the public-spirited work of the Duke of Rothesay, which is one reason I asked for this correspondence to be released.”

A spokeswoman for Prince Charles at Clarence House in London said: “The Duke of Rothesay cares deeply about the United Kingdom, and tries to use his unique position to help others. He has devoted most of his working life to helping individuals and organisations, to make a difference for the better of this country and the world.”

According to the spokeswoman, the Prince carried out over 600 engagements a year, giving him a “unique perspective” on issues he can help address. “Sometimes this leads him to communicate his experience or, indeed, his concerns or suggestions to ministers, from all governments, of whatever party, either in meetings or in writing,” she said.

“These letters released by the Scottish Government show the Duke of Rothesay expressing concern about issues that he has raised in public such as the preservation and regeneration of historic buildings and the benefits of sustainable food supply chains,” the spokeswoman added.

“In all these cases, the Duke of Rothesay is raising issues of public concern, and trying to find practical ways to address the issues.” The Prince was patron of the Soil Association, and had long championed food quality and provenance, she added.

Nonetheless, the Prince believed that he should have the right to communicate privately, she pointed out. “The publication of private letters can only inhibit his ability to express the concerns and suggestions which have been put to him in the course of his travels and meetings.”

The Scottish Government defended its decision to make the letters public. “It was entirely appropriate for the Duke of Rothesay to discuss issues of interest and concern with the former First Minister – and equally appropriate for the then FM to respond courteously and constructively,” said a spokeswoman.

“The Scottish Government has proactively decided to make these letters public, in response to a freedom of information request. Indeed, the former First Minister was strongly of the view that these documents should be made public in the interests of transparency, and made that view clear to the government.”