Michelle Obama hugs the Queen

The First Lady had only met the Queen for the first time earlier on Wednesday as she joined her husband Barack Obama to exchange gifts with the royals.

However, within hours the 45-year-old appeared to have struck up a close friendship with the Queen and even embraced her before leaving an evening function for world leaders.

The move is a departure from what is considered appropriate protocol when meeting the Queen.

Mrs Obama had curtsied to the Queen when she and her husband met her and the Duke of Edinburgh in the royal's private apartments.

Presents were exchanged between the two heads of state with the Obamas presenting the Queen with a personalised iPod containing a video of her visit to Virginia, Washington, in 2007.

In return, the royal couple gave them a silver-framed, signed photograph of themselves.

They then attended a reception at the palace for world leaders attending the G20 summit.

While guests began to file out at the end of the event, Mrs Obama and the Queen began chatting again and appeared to joke about their height difference.

As the pair moved closer, the Queen put her hand on the back of Mrs Obama who did the same for a few moments as they chatted.

Mrs Obama said a few moments later: "I really enjoyed our meeting."

Before the meeting, Mr Obama disclosed that the Queen was top of the list of things he likes about Britain, praising her "decency and civility" as important to people across the US.

When the President met the Monarch in a room used to stage audiences with foreign dignitaries, Mr Obama said to her: "Thank you so much for having us."

Addressing the Duke, he added: "It's a wonderful honour."

The two couples posed for pictures to record the occasion and the difference in height between the tall American couple and their Royal hosts was noticeable as they stood side by side.

The reception, held in the palace's picture gallery, was also attended by the Prince of Wales, Duchess of Cornwall, Duke of Edinburgh, Duke of York and Princess Beatrice.

At one point the Queen chatted for a few moments with Italian Prime Minister, Silvio Berlusconi as a few inches away the French President Nicolas Sarkozy held an animated discussion, through an interpreter, with US Secretary of State Hillary Clinton.

The guests sipped champagne and were served canapés, including mini Cornish pasties, smoked quails' eggs, foie gras and rolls of duck filled with melon.

The Duchess held a long conversation with Mrs Obama, while a few feet away Charles shared a joke the Brazilian President, Lula da Silva, and the Indian Prime Minister Dr Manmohan Singh.

As the drinks reception drew to a close, the politicians were ushered into the Throne Room for a group photograph with the Queen to mark the occasion.