She famously had a ‘complex’ relationship with Scotland.

But as a newly wed, Diana, Princess of Wales, told a school friend she loved being north of the Border and ‘playing with grown ups’ at Balmoral.

In a previously unpublished letter, dated September 27, 1981, to former classmate at West Heath Girls’ School in Kent, Katherine Hanbury, she also claimed to ‘hate’ London.

Although she adored exploring the Royal’s private estate in Royal Deeside, she found the strict protocol, intense security and rigid atmosphere of the castle exhausting and stressful.

But writing from Balmoral shortly after her royal wedding, the Princess painted a different picture.

She said she had enjoyed ‘endless sun’ and ‘calm seas’ during a cruise on the Royal Yacht Britannia.

Diana married the then Prince Charles when she was 20, having left school at 16 and dropped out of a finishing school in Switzerland before starting work in a nursery.

In her note, written on royal-crested paper and sent to her friend, she said: ‘We had a blissful honeymoon with endless sun and luckily calm seas. We are now up in Scotland until the end of October, which is a big treat for us - I adore being outside all day & hate London!’

She also added: ‘Its [sic] wonderful being married - I think its [sic] safe to say that after two months!’

The letter suggests she was adapting to her new life and role within the Royal family as she wrote: ‘Its [sic] a case of playing with grown ups!’

Diana and Charles had boarded the Royal Yacht Britannia after their wedding on July 29, 1981 for a 12-day cruise of the Mediterranean before heading to Balmoral for several months.

The letter is among a collection of items to be auctioned by Ms Hanbury.

It includes photos of the future princess at school. One shows her sitting with a number of friends, including the Scottish actress Tilda Swinton and film director Joanna Hogg.

The collection will go under the hammer at Gorringe’s Fine Art & Interiors sale in Lewes, East Sussex, in July. It has an estimate of up to £6,000.

The timing of this sale coincides with what would have been the couple’s 45th anniversary.

Albert Radford, books and manuscripts specialist at Gorringe’s, said: ‘This intimate archive offers a rare glimpse of Diana, Princess of Wales, before duty and fame had the final say.

‘She appears here as a young woman suspended between love and history - hopeful, unguarded, and not yet entirely claimed by the institution that would come to define her.

‘In these small, fragile traces, innocence lingers - along with a quiet stubborn belief in something as simple and elusive as love.’

The photographs include one of the Princess outside the art room, one of her in a block known as the ‘cowsheds’, and a third shows her standing outside, close to the playing fields.

Diana married Charles at St Paul’s Cathedral with an estimated 750 million people watching across the world.

They separated in 1992 and following the Queen’s intervention, their 15-year long union officially ended in August 1996.

She died in a crash in Paris the following year.