To anyone who has watched her perform her duties over a lifetime of public service, she is the picture of unflappable poise.

The secret of the Queen’s calm confidence can be traced back to a single piece of advice from her mother, a friend has claimed: to walk through the very centre of doorways.

Queen Elizabeth, the Queen Mother, had been acutely aware of her elder daughter’s natural shyness, Lady Penn said, and had gently coaxed her towards being as comfortable in public as her future job would require.

Lady Penn, former Lady in Waiting to the Queen Mother and friend of the Queen, said: “The Queen Mother told the Queen when she was very young to be brave.

“I think the Queen probably when she was young felt walking into a room full of people was rather daunting.