King Juan Carlos abdicates: Leonor, eight, will be Europe's youngest heir to the throne

The little girl currently holds an Infanta title, which is the form of address for direct descendants of the reigning monarch where they are not the immediate heir to the throne, and of which the male version is 'Infante'.

But Leonor will get a new title very soon and be classed as Princess, as her ex-TV reporter mum Letizia has been for the past 10 years.

This said, she will not be able to be sworn in to her position of future Queen of Spain until she is 18 years old.

And if her father abdicates or dies before she reaches majority age, it is likely the throne will be handed to the closest adult male blood relative of the soon-to-be King Felipe VI.

Although the Spanish Constitution does not provide for the abdication of a reigning monarch, it does include a section on swearing in as King or Queen and states that the subject should pledge to carry out his or her Royal duties faithfully, adhere to and ensure others adhere to the Constitution and laws of the country, respect the fundamental rights of the people, and of the 17 autonomous regions in Spain.

Princess Leonor will be expected to pledge all this, as well as declaring her loyalty and faithfulness to the King, her father.

Already, Leonor's short life has led to laws being changed – when she was born on Hallowe'en in 2005, the first child of the newly-wed Prince and Princess of Asturias presented a succession problem: until then, only male children could inherit the throne.

The then president, socialist José Luis Rodríguez Zapatero, arranged for the Constitution to be amended so that the first-born of the current heir, independently of gender, would be next in line.

This would prevent Leonor from missing her chance to be Queen if she later got a baby brother – but in practice, she acquired a sister when Sofía was born on April 29, 2007 and her parents have not had any more children since.

Leonor was christened on January 14, 2006 in the Zarzuela Palace by the Archbishop of Madrid, her grandparents became her godparents and, as is traditional, the water dripped on her forehead had come from the river Jordan.

She attends the private 700-euro-a-month Los Rosales school in Aravaca (Madrid) where her father went, just minutes from the Palace, and is described as 'calm and observing' and very like her grandmother, the outgoing Queen Sofía.

The Queen adores her grandchildren and spends precious holiday time with them – and like with her own children, only ever speaks to the girls in English.

As well as test-driving the Royal family's public website, Leonor has already undertaken 'public duties', appearing at official engagements with her parents, the first of these being the FIFA 2010 World Cup in South Africa when the whole family were present to celebrate Spain's victory.

The last time she was seen at one of these was on May 2 this year when, along with the imminent King and Queen Consort and her sister, she attended a military ceremony at the air base in San Javier, Murcia.

In accordance with Royal protocol, as she is directly in line to inherit the throne, Leonor will have to carry out a year's military service after leaving school.

She is the second-youngest of eight Royal grandchildren – Sofía, seven, her sister; the Infanta Cristina's children Irene, eight, but five months older than Leonor; Miguel, 12; Pablo, 13, and Juan Valentín, 14; and the Infanta Elena's children Victoria Federica, 13 and Felipe Froilán, 15.

Once she is officially Princess of Asturias, Leonor will also become Princess of Gerona, Princess of Viana, Duchess of Montblanc, Countess of Cervera and Lady Balaguer.

Her name has been very popular among Royals throughout Spanish history, particularly in the Middle Ages when several Infantas and Queen Consorts in the old kingdom of Castilla were baptised Leonor.