Tangle of successors to royal throne

Britain's Queen Elizabeth II. Photo / AP

The laws of succession to the British throne are as old as they are complicated. The Queen's four children and their offspring make up the immediate line, with Prince William's first-born to become third in line.

But those who may be eligible for the line make up a vast tangle of nobility and ordinary, untitled descendants from all corners of Europe.

Drawn up in an era of religious strife, the Act of Settlement 1701, which sets out the succession laws, says only Protestant descendants of Sophia of Hanover, King George I's mother, can accede to the throne.

Catholics and anyone marrying a Catholic are specifically excluded.

The monarchy's website lists 40 people, though some extensive counts reach about 2000 people eligible to inherit.

Several other European monarchs feature, led quite high up by King Harald V of Norway.

The first child of the Duke and Duchess of Cambridge will become the third in line to succeed the Queen - no matter if it is a boy or a girl.

He or she will follow Prince Charles and Prince William, pushing uncle Prince Harry down the order.

Arrangements had been put in place to ensure if a girl was born she would be a Princess and would not lose her place in the line of succession if William and Kate later have a son.

In centuries past, royal births were important moments for the monarchy and the nation as they were seen as assuring stability and continuity for the ruling family.

Although Britain's head of state knows the line of succession is secure, with many members of her family alive and in good health, the birth is still a significant event.

"If William and Kate weren't able to have children the throne would go to Harry," said Christopher Warwick, a royal writer and commentator.

"There will always be an heir to the throne, there will always be someone ready, that's the beauty of a firm line of succession."

The Queen's descendants are followed by those of her late sister, Princess Margaret, her only sibling.

The offspring of their father King George VI's siblings then follow, with more than 60 living people descended from his father, King George V, who died in 1936.

George Windsor, Earl of St Andrews, is the highest person currently excluded. A great-grandson of King George V, the 51-year-old would be 32nd in line, but married a Catholic.

The pending changes to the succession laws will remove the bar on those marrying Catholics and he will be restored to his place in the line.

His eldest child, Edward Windsor, Baron Downpatrick, 24, who was confirmed in his mother's faith, is the highest excluded for being a Catholic.

Besides Britain, it is also the line of succession to the thrones of 12 nations including New Zealand.

- AAP