AN independence campaign group has branded language used by Nicola Sturgeon in her resignation letter to the King as “deeply disturbing”.

In her letter, the former first minister signed off by saying: “I have the honour to be, Sir, Your Majesty's humble and obedient servant."

Sturgeon also wrote it had been “my pleasure to serve Your Majesty, Her Majesty the Queen and the people of Scotland”.

All Under One Banner (AUOB) took to social media to condemn the language in the letter and said it should be a “heads up” to self-declared republican and incoming first minister Humza Yousaf, who was selected by the Scottish Parliament on Tuesday afternoon.

Our Republic also said it was a “bizarre and anachronistic ritual that paints the Head of Scotland's Government as no more than a servant of the King".

READ MORE: Humza Yousaf selected to become sixth first minister of Scotland

While sharing a picture of a tweet saying Scotland should rid itself of “this sort of medieval sycophancy”, AUOB posted on Facebook: “Deeply disturbing language used here by Nicola Sturgeon – which is a massive red flag and heads up for Humza Yousaf that continuity won't cut it."

Our Republic added on Twitter: “A bizarre and anachronistic ritual that paints the Head of Scotland's Government as no more than a servant of the King instead of serving and representing the Scottish people, as the role is truly elected to do.”

According to Debrett’s, a London-based etiquette coaching company, any letter to the King or Queen Consort should be addressed to The Private Secretary to His Majesty the King unless you are known to the monarch.

For those who wish to communicate directly with the King, the letter should begin "Sir", or "May it please Your Majesty".

The first line of the letter should begin with the phrase “With my humble duty” – as Sturgeon did – while it should end with "I have the honour to remain, Sir, Your Majesty’s most humble and obedient servant”.

Yousaf will be appointed by the King as the new first minister tomorrow on the recommendation of the Scottish Parliament.