Ingrid Seward’s book Prince Philip Revealed claims The Prince of Wales campaigned for the SNP during a mock election while a pupil at Gordonstoun School in Moray. The royal expert says she was told about the incident by her late husband Ross Benson, a former classmate of Prince Charles.
She said Charles initially had “middle-of-the-road views and was never extreme about anything.”
She added: “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 in the book, she added: “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.
“Wearing his Stewart kilt, he marched up and down the grounds during the campaign shouting “Scotland forever, freedom for the Scots and down with the rule from Whitehall.”
“Together with his other political supporters, he held aloft a banner saying: ‘Vote for the Scottish Nationalists’.”
Ms Seward also claimed Charles went on to have a warm relationship with former First Minister Alex Salmond saying “they often shared a dram together.”
She continued: “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 OK by him.”
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The society, which was known as the “Sophist’s Club”, met at the home of the deputy English master Eric Anderson.
Ms Seward continued: “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.”
Ms Seward claimed on the last day of school, The Queen came to see him and “he wore his Hunting Stewart kilt in her honour.”
Prince Charles studied at the Scotland school between 1962 and 1967 and left with six GCE O-levels and two A-levels.
The Book claims campaigning for the SNP took place at the school’s mock elections ahead of the real General Election in 1964 which was won by Labour.
In response, a Clarence House source told Express.co.uk the claims are fanciful and entirely untrue whilst the SNP declined to comment.
It is a tradition that the Monarch is “above politics” and is politically neutral but is kept abreast of developments in Parliament.