As heir to the throne, he is forbidden from interfering in the world of party politics.

Yet Prince Charles once campaigned for Scottish nationalism, according to a new book that tells how he campaigned for the SNP in a mock election while a pupil at Gordonstoun School in Moray.

The teenage prince even went round the grounds shouting “Freedom for the Scots” and “Down with rule from Whitehall”.

Royal author Ingrid Seward, the editor-in-chief of Majesty magazine, was told about the incident by her late husband Ross Benson, who was a classmate of Prince Charles at the school.

She said: “Normally, Charles had middle-of-the-road views and was never extreme about anything. He prepared his speeches diligently but there was none of the quick flashes of inspired thought or sudden witty comments for which his father is famous.

“But one of the rare occasions when Charles blossomed occurred during the mock elections at the school. Charles became a vociferous supporter of the nationalists.

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“Wearing his Stewart kilt, he marched up and down the grounds during the ‘campaign’, shouting ‘Scotland for ever’ and ‘freedom for the Scots’. Together with his other political supporters, he held aloft a banner saying, ‘Vote for the Scottish Nationalists’.”

Ms Seward added: “Charles went on to have a warm relationship with Alex Salmond and they often shared a dram together.

“Mr Salmond, who also got on well with the Queen because of a shared love of horse racing, took a shine to Charles because he felt that anybody who loved Scotland was okay by him.”

“And through what Charles was as doing at Dumfries House, he saw that he really did love Scotland.”

‘But Charles has never had the he same relationship with Nicola Sturgeon. It is polite and civil but more professional. Maybe if she knew Charles once campaigned for independence the relationship would be warmer – and he’d be more embraced by his “fellow Nationalists”.

‘Given the current climate for independence, it is very amusing and a little embarrassing.’

Ms Seward added:’My husband Ross and Charles were members of the same debating society, the ‘Sophists club’, which met at the home of the deputy English master Eric Anderson ( later headmaster of Fettes and then Eton!). They were driven to house by Charles’s detective Michael Varney.

‘When Charles spoke in a debate all traces of shyness left him and he was one of the best debaters although Ross thought his views were a bit illogical but always clearly put. The subject of the monarchy was strictly taboo. But Charles prepared his speeches with notes and spoke convincingly.

‘He made lots of speeches supporting his party, the SNP, and a Tory supporter reminded him he was Prince of Wales not Scotland! “Freedom for Wales too” he said in the next election.

‘He loved Scotland loved wearing his kilt and on last day of school, the Queen came to see him and he wore his Hunting Stewart kilt in her honour.

‘He also loved Scottish poetry and Robbie Burns was one of his favourites and had a book of Scottish ballads he would carry around with him and learnt them by heart.”

Charles was a pupil at Gordonstoun from 1962 and it is believed his campaigning for the SNP took place at the school’s mock elections ahead of the real General Election in 1964, won by Labour’s Harold Wilson. The SNP’s leader at the time was journalist Arthur Donaldson.

It was during his term that the party began to grow and make its presence felt in Scottish politics.

An inspirational speaker, Donaldson enthused supporters while touring branches and constituencies.

The party won the 1967 Hamilton by-election, and polled more votes than any other party in the 1968 Scottish local authority elections.

During the 2014 independence referendum campaign, the Queen said she hoped ‘people will think very carefully about the future’.

The comment was made to a wellwisher outside Crathie Kirk near her Balmoral Estate in Aberdeenshire.

It followed reports that the Queen was growing increasingly concerned about the vote.

Royal officials insisted her remark did not breach her ‘constitutional impartiality, adding that ‘the monarch is above politics’.

Although Charles has reportedly described Gordonstoun as ‘Colditz in kilts’, he has also praised the school, stating it taught him ‘a great deal about myself and my own abilities and disabilities.

‘It taught me to accept challenges and to take the initiative’.

He became Guardian, or Head Boy, and left in 1967 with six GCE O-levels and two A-levels.

Ms Seward’s new book Prince Philip Revealed is out now.