The Queen was seen with US President Joe Biden and First Lady Jill Biden at the G7 (Image: POOL/AFP via Getty Images)

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The Queen met Joe Biden at the G7 summit yesterday making the 78-year-old her 13th US President and the monarch has long been used as the government's "secret weapon", according to a royal biographer.

In a display of soft diplomacy, the Queen alongside Prince Charles, the Duchess of Cornwall, Prince William and the Duchess of Cambridge were seen mingling with world leaders at the summit in Cornwall.

According to author Matthew Dennison, the Queen has remained seriously interested in Britain’s key ally since meeting First Lady Eleanor Roosevelt in 1942, when she was still Princess Elizabeth.

Since then she has carried out four state visits in the US and met every US President since Dwight Eisenhower, with the exception of Lyndon Johnson.

Barack Obama, pictured in 2011, said the Queen is "truly one of my favourite people"
(Image: PA)

Writing for the Telegraph, Mr Dennison says: "In almost every instance, the Queen has also inspired admiration and respect. These are the qualities that underpin her unofficial title as her government’s 'secret weapon'."

The Queen's charm offensive was first seen when she met Harry Truman, who called her a "fairy princess" in 1951, when ill health forced her dad King George VI to pull out of the trip.

Sixty-five years later, President Barack Obama called her one of his "favourite people".

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According to Mr Dennison, Bill Clinton said he found the Queen "very impressive", Eisenhower shared recipes with her following a state visit in 1957 and Ronald Reagan said hosting the Queen at his Santa Barbara ranch was a highlight of his presidency.

Throughout her reign, British governments "have used the Queen as a negotiating tool" as they deal with the US, says Mr Dennison, author of The Queen.

Bill Clinton and the Queen at Buckingham Palace in 2000
(Image: PA)

The Queen has met 13 US Presidents – including US President George Bush in 1989
(Image: PA)

He adds: "That every living former US president issued statements of respect following the death of the Duke of Edinburgh in April reflects not only American admiration for the Queen’s late husband, but affection and esteem for the Queen herself."

On Sunday, the Queen will host tea at Windsor Castle for President Biden and First Lady Jill Biden as the UK continues to roll out the red carpet for the US.

Mr Dennison said: "The cup of tea and Guard of Honour offered by the world’s oldest head of state to the representative of the world’s greatest superpower is a symbolic offering, token of a friendship that has been a cornerstone of British foreign policy throughout both their lifetimes."

On Friday, the Queen later prompted laughter from the Duchesses of Cornwall and Cambridge as she insisted on borrowing a ceremonial sword to cut a cake during a royal engagement at the G7 summit.

The Queen and President Dwight Eisenhower shared recipes after meeting 1959
(Image: PA)

She was handed the sword by Edward Bolitho, the Lord-Lieutenant of Cornwall, for the task of cutting a large cake marking the Big Lunch at the Eden Project.

When told by an aide that there was a conventional knife available, the Queen replied: "I know there is, this is more unusual".