She is one of the most famous women in the world.

But one of The Queen's former protection officer has revealed that even she is occasionally able to slip under the radar.

Richard Griffin described how Her Majesty had gone for a walk near the grounds of Balmoral dressed in tweeds and a headscarf.

So low key was her look, that a group of American tourists completely failed to realise who she was.

Mr Griffin, who was speaking at a recent talk about his time working with the royals, said the Queen was keen not to blow her cover, according to The Times.

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The Queen's former protection officer has told how the monarch once walked near the grounds of Balmoral completely unnoticed - because she had left her tiara at home, and was dressed in tweeds and a headscarf

Her Majesty's trips to Balmoral Estate in Aberdeenshire, Scotland, are a time when she enjoys a more low-key way of life, donning wellington boots and driving around the estate in a mud-spattered Land Rover

She played along when the American tourists asked: 'Do you live round here,' simply replying that she had a house nearby.

And when asked if she had met the Queen, she simply gestured to her companion and said: 'No, but he has,' according to the policeman.

The Queen was completely unrecognisable to the tourists, who would have been more used to seeing the monarch in a bright array of colourful suits and hats.

Countess Sophie of Wessex recently explained the reasoning behind her colourful outfit choices, revealing: 'Don't forget that when she turns up somewhere, the crowds are two, three, four, ten, 15 deep, and someone wants to be able to say they saw a bit of the Queen's hat as she went past.

'She needs to stand out for people to be able to say 'I saw the Queen.'

Her trips to Balmoral Estate in Aberdeenshire, Scotland, are a time when Her Majesty famously enjoys a more low-key way of life, donning wellington boots and driving around the estate in a mud-spattered Land Rover.

Last year, a story emerged about Crown Prince Abdullah of Saudi Arabia's visit to Balmoral in 1998.

The Queen (pictured at the Imjin Barracks, Gloucestershire in November 2015) was unrecognisable to the tourists, who would have been more used to seeing the monarch in a bright array of colourful suits and hats

Sophie, Countess of Wessex recently revealed: 'Don't forget that when she turns up somewhere, the crowds are two, three, four, ten, 15 deep, and someone wants to be able to say they saw a bit of the Queen's hat'

Writing in his memoir, Ever the Diplomat, Saudi ambassador Sherard Cowper-Coles recalled that the Queen had asked her guest if he would like to go on a tour of the estate.

'An initially hesitant Abdullah agreed,' he said. 'The royal Land Rovers were drawn up in front of the castle. As instructed, the Crown Prince climbed into the front seat of the front Land Rover, with his interpreter in the seat behind.'

'To his surprise, the Queen climbed into the driving seat, turned the ignition and drove off. Women are not - yet - allowed to drive in Saudi Arabia, and Abdullah was not used to being driven by a woman, let alone a queen.