Why Melania didn't curtsy to the Queen

US President Donald Trump and wife Melania have begun their three-day state visit to the UK, and already royal watchers are questioning their behaviour.

The Trumps were welcomed by Queen Elizabeth II at a grand ceremony at Buckingham Palace Garden on Monday, with the moment captured on camera.

Royal watchers are used to seeing members of the Royal Family and other dignitaries bow and curtsy to the Queen.

However, the President and First Lady chose not to do this, opting instead to shake hands with Her Majesty.

While this may prompt accusations of 'breaking protocol', the Royal Family's official website indicates that isn't the case.

The website states there is "no obligatory codes of behaviour when meeting the Queen or a member of the royal family", but notes "many people wish to observe the traditional forms".

The guidelines state that if men choose to bow, they should do so from "the head only" while women should perform "a small curtsy".

"Other people prefer simply to shake hands in the usual way," it continues.

In the Trumps' defence, when former US President Barack Obama and wife Michelle visited in 2011, neither bowed or curtsied to the Queen, shaking hands with her instead.

The President and First Lady touched down at Stansted Airport on Monday morning.

They then boarded a helicopter which took them to the US ambassador's official residence in London, Winfield House in Regent's Park, where they are staying.

The couple then travelled via car to Buckingham Palace for the ceremonial welcome.